Dharma sells their own brand of Procion reactive dyes that come in a bazillion colors (although you really only need 3, extra colors come in very handy to avoid the tedious and time-consuming mixing). They also sell clothing blanks (various garments, white cotton, rayon or silk) for super cheap -- between $4 and $2 a t-shirt, depending on how many you buy. The Gildan shirts are very very nice, by the way.
I'm also going to throw in here; I'm really not a tie-dye person (uhm, to say the least); my recent crafty fetish is doing stencil work (Han Solo: http://www.livejournal.com/users/thespian/838670.html. Calico Jack Rackham's pirate flag: http://stencilrevolution.com/photopost/showphoto.php?photo=39466 ).
Anyways, what I wanted to bring up is that you can get all SORTS of great shirts in any colour, from S through 5x, for about $2 Canadian a shirt, along Spadina in Toronto - Toronto's Chinatown. I'd highly recommend, if any of you live in areas with a Chinatown, you look into it; there are about 15 dealers that I know of between Dundas and College here, and the shirts are really very good, especially for the price. They had long and short sleeves there. S.