As Chris said, Rit Dye is a fabric dye and just look for it at any general department stores or grocery stores.
If the spray paint works, I would use that. (First time I've heard of that, I'll keep it in mind). Depending on the size PVC to be dyed, you need a pot large enough to stick it in the boiling dye, and the boiling water WILL warp the PVC to some degree. I've had some pieces unusable and had to start agian. But the end result is worth it if your willing to go though it.