AFAIK, AEPhi nationals doesn't run a store
but I went to school in the biggest and best college town in the world
and we had a pretty good store where we could buy stuff.
It's easy enough to make your own letters, though... just buy a nice T-shirt or sweatshirt, go to your local fabric store and pick out something you like, buy the material (you don't need much), cut out your letters, and stitch them on. It's a lot cheaper than going to a Greek store, and fairly easy if you have any sewing skills whatsoever.