There is one online / in store company that tries to keep stock of many GLOs para, Joe Toga,
www.joetoga.com. I actually visited their store and they did have a lot of stuff for us.
As far as greek stores in particular, it has been my experience that you need to get to know the owner / buyer and encourage them to keep our stuff in stock. The 'catch' is that you have to encourage your other sorors to buy from that store so that the owner / buyer knows that your request to keep our stock up to date was serious.
I know of a few greek stores who order their stock based on the number of members on campus / in town.
There is this one greek store in Houston and the owner refuses to keep a lot of SGRho stuff in stock because we ususally don't have large lines at the two college campuses near his store, UH and TSU. But then there is this Delta who has a greek store and she does make an effort to keep our stuff in stock, although she has moved her store again.
