I'm from GU, and have been to build a bear once in my life, but I thought the fine folks at Build A Bear offered Greek Lettered shirts?
If you go to this link:
http://www.buildabear.com/acb/showde...CATID=22&emb=1 , and scroll a little bit it says you can select up to three Greek Letters.
I'm not sure if this is where Alpha Omicron got them, but I hope it helps