I usually buy them at my local Walmart. Here's a link where you can order them online.
http://www.walmart.com/ip/Shout-Colo...24-ct/11046026
I have washed all sorts of colors together and I usually toss in two or three if they is great contrast in the colors I am washing. I am always amazed as to how they do not run. The color catcher pieces are always full of dye. Hope it works for you.