Cathy
First off, I have not personally used any of the following online stores, so I can't vouch for how good/bad they are. But here's a link from my chapter's website that list a lot of Greek resources (that often have links to merchandise stores) and stores. Hope this helps!
http://homepages.go.com/~phiplayground2/greeklinks.html
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Phi Beta Fraternity for the Creative and Performing Arts.
http://homepages.go.com/~phibetaphichapter/index.html