I really enjoy this site; there are a lot of great titles for sale here.
Lots of books on religion and the occult, and there are some titles here I haven’t seen elsewhere.
Some are out of print, some are first editions, some are just interesting so one should be able to find some gold here, no matter your preferences.
https://www.weiserantiquarian.com/
If the classics are more your thing I would recommend “Project Gutenberg”, where you can find tons of them for free.