I LOVE H.P. Tops.
http://www.hptops.com/location.htm
We've gotten some really superb T shirts from them. They're based in Chicago, but they accept orders online and by phone. The shirts we got really fit nicely.
Also, if you do a search for greek clothes, you'll find tons of companies. I personally like Greekave.com because it's really inexpensive, but good quality.