It's been a year since I last did business with Mammoth Printshop since I really had the need to print more shirts, but they're really good. Cool peeps!
I would check out both Zazzle and Spreadshirt. We just got out shirts from Zazzle for SXSW and the quality is really good!
I'll second Spreadshirt. I've bought a number of shirt from them and I'm happy with the result. The price is pretty good, too.