What is stopping you from being a sign shop? Why not just make “signs in the shape of a box for the purpose of keeping animals”? What’s the worst that could happen
So another, possibly better option, is to find a place within 100 miles or so and order 10 to 20 sheets at a time so you won’t have to make the trip as frequently. Would probably be annoying to have to make a longer trip but if you buy big enough quantities you would only have to make them every couple of months (depending on how much you need and use).