My library has book sales about once a quarter and you can fill a whole bag for $5. They also have a “shop” which is open all the time and books are usually $1-2.

I also like ThriftBooks and have ordered a lot from them with no issues.

2nd and Charles is also a fun place to shop for books and they have used and new. Surprisingly, Ollie’s (an outlet store) has a very good book selection for very cheap. They don’t have an online presence, so you have to have one nearby to frequent or maybe take a road trip!

I use Amazon as a last resort for books I haven’t been able to find anywhere else.