I love to scavenge Goodwill for white, tie dye-able stuff for me and the kids. The results are always one of a kind, and also cheap, which is a plus.
(If you're new to tie dyeing, you'll want to check out Dharma Trading Company, for the best dye you can buy, as well as the details on how to get started.)
When thrifting for stuff to dye, you'll be looking for cotton, rayon, linen, silk, or some combination thereof. No synthetics.
You''ll also want to watch for stains. If you're going for a bright, multicolored effect, you can probably cover up most stains, but if you're going to be dying the item a single color, or a couple of light colors, the stain might still be visible.
Here are a couple of Goodwill shirts I've recently dyed for myself.
Pretty cool, right, as long as you don't count the awful pictures. :)