I make my own tie dye shirts all the time. In fact I'm wearing one, but yeah, go down to any arts and crafts store, and buy some dye, the instructions should come in the box. And of course a white shirt.
Hey, now that you mention it. Maybe ask 'drumminmama', the mod --I think she's got some linx. There are good ones & bad ones. It's an artform! Tie-dying is a big deal. Art is sold in a Gallery, not Wally-world. No, don't buy that crap. Save it for the good stuff man...
Learn the secrets of tie dying and create your own beautiful fabrics. http://www.kinderart.com/textiles/easytiedye.shtml