Ah, the million dollar question!
It really depends on the product and how easily it is stored and created. The stock you have on hand is highly depend on how what sell and what your audience is. I usually have 3-5 copies of select items.
As for hats, maybe have 2-3 of each design on hand and if you sell out--that's awesome.
And since this is in Downtown, there is are several place with in blocks for printing. I will be sure to ask our Cartographer to place such information for printing on the convention maps.