DARK SOULS II: Scholar of the First Sin

If anything is wrong or any vital information is missing, please feel free to comment and I will edit the solution.

Where to find the DLC keys:
Frozen Flower (Ivory King): Located on a skeleton in Drangleic Castle, in the room after the soul doors with ruin sentinels behind them.
Dragon Talon (Sunken King): Behind a locked door halfway down the Majula pit. You'll need the key from the giants in the Black Gulch. Use Gilligan's longest available ladder for easy access.
Heavy Iron Key (Old Iron King): In the salamander area behind the door just before the Last Giant in Forest of Fallen Giants. The key can be found in the first room of the Iron Keep.

Merchants you will need to find for this achievement:

Merchant Hag Melentia - First encountered next to the second bonfire in Forest of Fallen Giants. After you exhaust all her dialogue here, she will move to Majula the next time you leave the area, where she can then be found sitting by the broken wall near the bonfire.

Licia of Lindelt - First encountered by the bonfire after the Dragonrider fight in Heide's Tower of Flame. Moves to Majula after you exhaust her dialogue.

Cromwell the Pardoner - Right after you come out of the Prowling Magus + Congregation boss fight in Brightstone Cove Tseldora, turn left and climb the ladder in the little alcove to find this guy. He never moves from this spot.

Straid of Olaphis - Petrified in a cell in Lost Bastille, up the stairs from the room on the Bastille side of the bridge to Sinner's Rise. He never moves from this location.

Chancellor Wellager - Right inside the gates of Castle Drangleic, a mandatory area. Just go up the stairs and he's right there, in ghost form. He never moves from there, but you will need to go to New Game++ to access some of his wares.

Blue Sentinel Targray - Found right after the Old Dragonslayer boss fight in Heide's Tower of Flame. You will need to possess a Token of Fidelity before he'll speak with you, which can most easily be found by dropping off the bridge leading to the Undead Purgatory in Huntsman's Copse, and collecting the item there.

Stone Trader Chloanne - Found at the start of Harvest Valley. Exhaust her dialogue to make her move to Majula.

Grave Warden Agdayne - In Undead Crypt, shortly after the room with the mages after the first bonfire. When you get near his room, he will warn you to put out any lights you have with you. Do this, or he will turn hostile. As long as you remain in the dark, he will talk and sell things to you.

Felkin the Outcast - Found in a chair by the first bonfire in Huntsman's Copse. He never moves from there, and you'll need at least 10 FTH and INT before he'll talk with you.

Head of Vengarl - Found on a pile of rocks in the Shaded Woods (foggy part). Hug the left wall when you enter from the crossroads and you'll find him. He never moves from there, for good reason.

Now for the actual miracles:

Heal - Bought from Melentia/Licia (1500 soul cost).

Med Heal - Bought from Licia (3000 soul cost).

Great Heal Excerpt - Purchase from Licia (4500 soul cost).

Great Heal - Bought from Cromwell (4500 soul cost).
Found in the big circular room at the bottom of the Majula pit, the one with a bridge across it. It's on a body on the lower level.

Soothing Sunlight - For this, you will first need to find the Crushed Eye Orb in the Undead Crypt. After you pass through the room Agdayne is in, drop off the ledge outside, to the left. Then climb up the nearby ladder and open the chest. Now go find Licia in Majula, and use the orb near her. You will invade her, and when you kill her you will get this miracle.

Replenishment - Bought from Licia/Cromwell (3000 soul cost).

Resplendant Life - Bought from Licia/Straid (4500 soul cost).
Can also be found in the cell next to Straid's in NG+ onwards, for some reason.

Bountiful Sunlight - Bought from Wellager in NG++ onwards (11000 soul cost).
Or reach rank 3 in the Blue Sentinels Covenant. Just don't.

Caressing Prayer - Bought from Licia/Cromwell (2000 soul cost).

Force - Bought from Licia/Cromwell (1800 soul cost).

Wrath of the Gods - Bought from Wellager in NG++ onwards (8200 soul cost).
Or reach rank 2 with the Blue Sentinels. Again, just don't.

Emit Force - Bought from Cromwell/Targray (4200 soul cost).
Found in No-man's Wharf, up on the top cliffs near the house with all the spooky monsters.

Heavenly Thunder - Bought from Cromwell/Targray (3300 soul cost).

Lightning Spear - Bought from Licia (6000 soul cost).
Found in Earthen Peak. Drop off the platform just outside the door from where Pate sits, and you'll find a hidden chest there with the miracle.

Great Lightning Spear - Bought from Straid, only after defeating Velstadt in Undead Crypt (13000 soul cost).
Found in Undead Crypt, behind an illusory wall in the room you drop down into through a circular hole.
Found in the same room as the Dragon Talon (see start of solution).

Sunlight Spear - Awarded for reaching rank 3 in the Heirs of the Sun covenant. The altar is in Harvest Valley. No two ways about it, it's gotta be done. 30 Sunlight Medals must be offered. There are three ways to get these.
1. Summon other Sunbros to help you kill a boss or kill enough enemies, and you will get one medal for each member present. Their summon signs are more bright yellow than others.
2. Join others as a Sunbro, and you will also get a medal when you beat a boss or kill enough enemies. I recommend placing your sign before the double Dragonrider fight in Drangleic Castle.
3. Farm them offline. This is not fun, but if you really want to, it can be done. You'll need a lot of Bonfire Ascetics, and preferably Item Discovery+ gear, like the Jester Hat, Symbol of Avarice, Prisoner Tatters and the Covetous Gold Serpent Ring (get the +2 version from killing Belfry Gargoyles on NG+). The enemies to farm are Falconers, and there are three spots to do it, although only one is good. The first is Things Betwixed on NG+ onwards, as four Falconers always spawn right at the start. Downside is that the run from the bonfire is quite long. The second is the Royal Campsite at the start of Brightstone Cove Tseldora. Plenty of Falconers here, but you'll have to kill the Congregation every time you want to use an ascetic. The third and best option is to use the bonfire at the crossroads in the Shaded Woods. Heading left here (towards the castle) you'll encounter five Falconers every time. Two just outside the door and three past the tower. This bonfire can be asceticed (?) without killing a boss, as long as you've killed Najka on the current playthrough. The drop rate is terrible even with maxed item discovery, so prepare to spend hours doing this if you choose this method.

Soul Appease - Bought from Chloanne/Agdayne (8800 soul cost).

Blinding Bolt - Trade the Old King Soul with Straid (10000 soul cost). The soul is acquired by killing the Old Iron King on NG+ onwards.

Magic Barrier - Bought from Vengarl/Felkin (2700 soul cost).

Great Magic Barrier - Bought from Wellager (9300 soul cost).
Found in Aldia's Keep. In a chest in an alcove on the right side of the second floor of the first big room.

Homeward - Bought from Licia (2400 soul cost).

Guidance - Bought from Licia (3700 soul cost).

Sacred Oath - Trade the Soul of Velstadt with Straid (3000 soul cost).

Unveil - Bought from Straid (2200 soul cost).

Perseverance - Bought from Cromwell (3500 soul cost).

Sunlight Blade - Bought from Straid, only after defeating Velstadt in Undead Crypt (12400 soul cost).
Found in Shrine of Amana. After the second bonfire, take the narrow path to the right by the big cyclops troll all the way to the end to find this in a chest.

DLC miracles:

Sunken King:
Denial - Found in the same room where you get the Dragon Stone. It's a room with lots of spikes on the ground, which can be removed by firing an arrow at a switch on the upper level. This room holds one of those big circular doors, and you'll have to fire several arrows at the switch behind it to make it spin into the right position. Inside is a chest with the miracle.

Ivory King:
Splintering Lightning Spear - Shortly before you get the Eye of the Priestess, you'll find yourself on the outer parts of the ramparts, with a staircase leading down and onwards, close to the bridge you initially arrived into Eleum Loyce on. Don't go down there. Instead, go up to the big tower on the upper level, and climb an invisible ladder there to find a chest with the miracle. This ladder becomes visible after you dethaw the area later on, but you can still climb it before then.