www.GreekAve.com has some really great prices. My mom got me a fleece jacket for Christmas and it looks great!
You can order from any of the big sites (greek101, greekgear, etc), unless you want something screenprinted. For screen printing try a local place that does shirts for sports, clubs, etc.
We used to get our done by a fellow Greek and close friend of ours at NJIT.
www.illtema.com