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 Born machine. They give you items to use in Earth Bound. Access the menu to see a list of Pixballs you've collected, and you can also see what item each one gives you in Earth Born (Mad Rat gives the magic keys needed to access blocked areas in Earth Born).

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 really can wait until the beginning of Chapter 7 to start collecting if you want, or you can spend your tokens as you progress through the game.

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 be able to complete this achievement at this point, no matter how many you are missing. At this point 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.

To complete the entire set, you will need to beat Earth Born, as the last 4 Pixball characters (3 Dragons and the Pixel King) are not available in the machines until you do. You will receive the Fat Dragon Pixball in story progression, while the other 3 Dragon Pixballs are obtained from the machines. However, you will not see them as available, they will just be obtained randomly. I added my strategy to completing Earth Born as a guide, click this link if you need help with that -