HOw many do you want? If you're only doing 20-30, your best bet is
CafePress and click on the "make your own" link at the top of the page (or something like that).
If you want to buy hundreds, I have some links to cheap places, but I will need to figure out what they're filed under. I have been thinking of doing some in bulk, but the CafePress deal is too good if you don't really want to get into stocking and sending t-shirts. CafePress is pricey on a per shirt basis, but when you consider that you have no stock, no overstock, can just log in and send your shirt to anyone direct from CP and so on, I think it's a good deal. They handle customer service and returns and everything.
If you think you're the kind of company where people might actually buy your logo shirts (sell performance car parts or organic coffee, or something people identify with), then it might be worth it to create a free CP shop and sell your shirt for a breakeven price. It would be like completely free, hasslefree marketing if people actually purchased.
Word of warning: do NOT sell black t-shirts. I just ordered one of my own that should have been good (black and white design) and it was total junk. Everything else has been great quality, but the black shirts are in beta and it is not ready for public release yet.
One last thing - you can upload one high res design and get stickers, fridge magnets and everything at CP. It's fun actually. I put up some designs to give to friends, so it's a total surprise to get $100/month out of
CafePress. I mostly take my money and buy my own stuff to send to friends. It's cheaper fun than lots of other things and if you have a couple of decent designs, you can make a few dollars.
I don't get the ThreadlessTees thing. I can't believe people are willing to give away their designs for free like that. Total BS. I have some designs that sell a bit here and there, so why exactly would I give them away for the chance of $1500 when I can let them sit where they are and give me about $100/month every month? If I went to the trouble to promote, possibly more.
To me threadless is a total Tom Sawyer ploy - painting fences is so much fun, I'll let you do it for free if you want. Creating designs is so much fun, we won't even charge you to submit your designs and let us sell them without giving you a cut. I can't believe that people are so willing to give their hard work to some company.