ANSWER: AT A LIBRARY!
Libraries are a treasure for many reasons, not the least of which are great sales. Our first purchase at a library occurred in Menlo Park, California during the 1980s. We bought a nineteenth century edition of The Saints’ Everlasting Rest by Richard Baxter. It cost 50 cents! Sure, it was a bit beat up, but such a deal.
Since then, I’ve always tried to visit libraries, especially when I’m traveling. The picture below shows five great books I recently bought in Florida for $1 each. All are unmarked and ready for my own system of note taking.