Online purchases available at the RIT dye website:
http://www.ritdye.com/Rit+Products+Available.5.lasso
If you buy local, be careful. Not all dyes are the same and there is another competing brand that's usually locally available, but doesn't work as well. So, make sure the box says RIT.
Even then, some plastics/nylons don't dye well. I know for a fact that the plastic/nylon type used on those whit-ish cutting boards (and like the counter at SubWay) do NOT take dye well. The best you get is a tint, and looks crappy.