I also got mine at an art supply store, and have seen it at Michaels. You have to look in the painting section. It is often used with watercolors.
I mix mine in those stackable pill boxes (the round ones) that they have at walmart (or any drugstore) for that matter. That way I can neatly store them when I'm not using them.
Here's a link to one just to see what I mean:
http://www.drugstore.com/products/pr...UY-PLST-0-SRCH
I got mine at walmart, and they come with little white labels, so you can write on the outside what color you have.