I stopped using Amazon a while back, but it was where I got all my books for a long time. I do thriftbooks mostly now, and try to buy directly from publishers when it’s a newer book, but I’m always interested in finding new spots to cop some sweet books.
2nd and Charles if you have one near you. Also Book Exchange. But those are very location specific.
I love second and Charles. Lots of decent books for a good price, plus I get to gawk at their overpriced retro games and occasionally walk away with a genuinely good deal, like when I got halo 2 for $5 to replace my long since lost steelbook from my childhood. Mines pretty far though, so I rarely go.