Eastward

You will collect Tokens from treasure chests during the game. Use these at certain machines to obtain different Pixballs. The first one I found was in Greenburg, next to the Earth Bound machine. They give you items to use in Earth Bound, and I believe the rare ones contain the magic keys needed to unlock inaccessible areas in Earth Bound.

Each Pixball machine should show which ones it holds, and the Pixball machine prizes will change after you collect the 6 that are shown on the machine, or if you reload a save. You can collect them as you progress through the game or wait until Chapter 7 and do it all at once (see last section).

The Fat Dragon is given to you automatically through story progression, but you will need to beat Earth Born to unlock its availability (along the last 4 on the list and their alternates) in the Pixball machines. You will not be able to see the other 3 Dragons in the Pixball machine prize tiers, they will just be obtained randomly. If you need help beating Earth Born, click this link to check out my strategy -

Thanks to Dunge2 for adding every Pixball has an alternate color, so really you need to collect 40, not just 20. This needs to be done before completing the game as the alternate colors are not accessible in chapter select.

There are 50 tokens total available found in treasure chests. You can purchase tokens in Chapter 7 in Ester City from a robot, located in the upper left corner of Wisdom Street. Buying tokens from the robot costs 25 salt per token. You will have a chance to farm salt shortly after arriving in Ester City, so you can complete this achievement at this point, no matter how many you are missing. One of the enemies there is a skeleton dog-like creature that drops 10-30 salt per kill so farming a lot of salt is easy. I farmed 5000 salt, spent 2500 on 100 tokens and was able to finish my collection with plenty of tokens left over.