Due to internet ordering, it’s not so much what is unobtainable here compared to the US, it’s the price. Having said that, I do sometimes see bottles reviewed that aren’t usually available over here. Perhaps a distillery visit would provide the opportunity to buy a unique bottle. Bourbon is generally much cheaper in the states than here. But if you get clobbered for import duty on your return then that would wipe out any gains.
You don’t say where you’re travelling, but I was in San Francisco a few years back and was very disappointed at the duty free shop at SF Int. Airport- pricey and very limited selection. Better to buy in the US and take your chance with customs. IIRC you’re limited to one litre of spirits in any case.