I see a used gibson T-shirt on Ebay, so there must be somewhere to buy them, or would a person have to go to Nashville to find them?
The only place I knew was the Gibson store at Oprey Mills. Now that Gibson's outta there, I don't know where you could go. Maybe an online search is in order.
I just did an online search, and there's a lot available online.
At a local Guitar Center I noticed Gibson T's and ball caps.