Synthrapol is a type of liquid soap. I will link to it on Amazon:
http://www.amazon.com/Craft-Synthrap...dp/B000YZ3UHQ/
Some quilt shops carry it, and it is widely available from onlines sites such as DharmaTrading and Pro-Chem.
Synthrapol is stronger than color catchers. Color catchers are pieces of treated paper that you put into the wash to catch vagrant dye particles. I use color catchers whenever I do a load of my dd's black clothing, as black always seems to give off some dye. These are sold in a small box in the laundry section of Walmart, grocery store, etc. Here again is a link to color catchers on Amazon:
http://www.amazon.com/Shout-Catcher-...dp/B0000DIWJF/
In your particular case, I would strongly recommend using both Synthrapol and some color catchers (encased in a mesh bag). Just remember that domestic front-loading washing machines do not use enough water for Synthrapol to be effective. If you have a domestic front-loader, it would be better to do the first wash with Synthrapol at a laundromat. Also, be aware that Synthrapol requires hot water to work.