It really depends what you're looking for exactly.
The traditional JD t-shirts are pretty easy to find, a million and one places on the internet do them, and they can be picked up in pretty much any flea market these days.
I think going for something *slightly* different might be better.
For example this shirt from Savant Mens:
You could forget about Jack Daniels altogether and opt for a less "mainstream" Whiskey like Bulleit:
