Try
http://designergreek.com/design.shtml. I have never ordered from them but a lot of people on GC have. There is an old thread on them and how you can see the letters before you order them.
Also check out
www.greekbox.com. It is owned by Doug, the guy who used to own Fraternity Row. They've done a lot of our Conferences and Conventions over the years and are pretty reliable.
Laura