A second for "Don't waste your time trying to dye poly<whatever> fibers. They won't take it."
In my experience, the best place to purchase dyes is
Dharma Trading Co. Homepage - no matter what fibers you try to dye. These are much, much better than Rit when it comes to color choices, color brilliance, durability, and what they do to the fibers. I have sworn off Rit after discovering fiber-reactive dyes, it's one of my preachy issues ;-)