For T-Shirts and clothing I think the website's name is
http://www.customink.com
Check it out. If they don't have your crest on file then you can upload it on the spot and design the shirt to your liking. The chapters I've talked to that have used it have been very impressed with their quality and pricing.
Otherwise, just shop around online greek merchants, there are lots that are very willing to do custom jobs. Usually if you have a bitmap of something (or jpg or whatever) they'll be able to produce whatever you want!
Best of luck to you!
LHT
Kevin Taylor
Mu Tau 5
University of Central Oklahoma