I use the Shout dye catchers, which are one-time use. They come in a small box in the laundry area of Walmart, grocery store, etc. Here's a link to it on Amazon:
http://www.amazon.com/Shout-Color-Ca.../dp/B0000DIWJF
The box is only about 6x8 inches and 2 inches deep, so it can be easy to miss when you are looking up and down the laundry aisle. Once I fnally located them, they've been easy to find.
I just wanted to add that dye-catchers are not a total substitute for Synthrapol. The dye-catchers work fine when there isn't too much dye to be caught.