Saints Row 2

This solution is not mine, but it helped me get the last 4 cars I needed. It is so good, that I wish I had found it earlier. The credit for this solution goes to user ViperYourMother on the GameSpot Saints Row 2 forums.

Here's a trick that I've now used on 6 different hard-to-find cars:

1. Go to the district you need (Like Projects for Mag)
2. Enter and start shopping at a store (Like On Thin Ice). You don't have to buy anything, just hit Y to shop, and then back out.
3. As soon as you walk OUT of the store, you should see the yellow car icon driving around 9 times out of 10.

Just complete the lists from the five chop shops in the game, delivering the cars to the nearest chop shop to where you find the car.

To help you in your journey, complete the Suburbs Fuzz activity to have your police notoriety reduce quickly. On some of the cars, it can make a one-star wanted level disappear into nothing and a three-star go down to a two quickly, which really makes a difference, especially when you go to get the weakly-armored, slow Delivery Truck from Saint's Row.

If you are having trouble jacking cars for the lists, either create a road block with a large vehicle such as a bus, or take careful aim with a pistol and kill the driver, both of these tactics will be hard to achieve if they are not done right the first time you try--if you fail to stop the car or its driver, the driver will freak out and if you cannot catch up, the game will make the car disappear since it vanished from the view of the camera.

And remember to SAVE OFTEN. This mantra can't be repeated enough with Saints Row in general, but especially since the game does not autosave at all during this activity, it's important to save either after every delivery or after the really tough ones.

Other tips:

(Thanks Gnemlock!) Steal cars and store them in your garage, which may be closer than the nearest Chop Shop and can be delivered with a simple phone call to the car delivery homie.

(Thanks Spoilt8!) Complete all levels of insurance fraud -- this unlocks unlimited sprint, giving you a better chance to keep up with vehicles.

Ok thought i'd put a list of ALL the cars required for this achievement, they can be delivered to ANY of the Chop Shops. Obviously you need to get the list from each of the 5 Chop Shops before the vehicle can be delivered.

Here's the list.

Suburbs Chop Shop:
Mockingbird
Churchill
Hammerhead
Compton
Topher
Quota
Five-O
Ambulance

Downtown Chop Shop:
Go!
Taxi
Wellington
Alaskan
Varsity
Zenith
Justice
Titan

Apartments Chop Shop:
Swindle
Betsy
NRG V8
Raycaster
Melbourne
Bezier
Cosmos
Status Quo

Truckyard Chop Shop:
Voxel
Magma
Attrazione
Superiore
Venom Classic
Eiswolf
Socialite
Peacekeeper

Factories Chop Shop:
Voyage
Danville
Mag
Bag Boy
Backhoe
Bulldozer
Delivery Truck
Longhauler

You can either horde cars in your garage, and then have your homie deliver them for you, jump into random cars and hope their on the list, or if you select a car from your start menu, it will show up on the map if it's near, and like in Trout Juice's solution you can enter and exit a store in the correct area until the car shows up on the map.

EDIT: Thanks to A7X Jamie UK for reminding me the you can customise your gang vehicles, choosing vehicles you need for the chop shops, your gang will then drive around in these vehicles, making them much easier to obtain.

What I have been doing is using a tow truck. Find a vehicle, and back into it even if its driving, and then tow it to the location. You will get no police action until you drop the car and get into it to drive it into the marker.

The driver will usually exit just after you back into a car, so you don't need to worry about that.

If it glitches, start a new game, and just do chop shop...I left the main vehicles taxi & ambulance for last, they're everywhere and can found easy. Deliver the ambulance/taxi & it should pop