I love hunting for 2nd hand everything, but especially books, so the somewhat lower hit rate for quality stuff in most places never really bugged me. If you're doing it just because you happen to be where used books are and you like poking through things the hunt is it's own reward, and so on.
Besides the standard classics like dedicated used book stores and the nicer Goodwills and so on, the main place I have a lot of success with good finds is bigger donated book sales thrown by libraries. I got lucky that I grew up by one of the biggest in the US that had a huge sale twice a year I could regularly hit up, but most everywhere has at least a few and you can always check book sale finder (which looks like a refuge from 90s era geocities, but is actually good) to check out what's coming up in your area.