When traveling (and I count traveling even as going downtown for a day!
) I always have at least 300 books with me. That avoids the immediate "gotta-have-this-book-NOW!" thing, as I always have plenty to read. I'd simply make a note of the book and get it when I'm back home.
But in case I really
really need it right then and there that day, when out of town I always have my netbook with me, which is loaded with Calibre and Uncle Alf, so once I got back to my netbook, I could hunt down the best price and sideload it to my reader. And that's barring any problems trying to buy through my US accounts if traveling overseas.
But I really can't imagine anytime I couldn't wait until I got home, unless it was a much needed translation dictionary maybe depending on where I was.