If you like classics at all you can download a lot for free from sites like Gutenberg.

This isn't one I do but both Kobo and Kindle also have a monthly subscription service where you get access to a lot of books. It might depend what kind of books you like but I know some people who get a lot of books that way.

Keep an eye out for sales. I'm subscribed to Kobo's email newsletters. A lot of books on my reading list have gone on sale for really cheap and I found out because of those newsletters. A lot of big bestsellers go on sale eventually too.

Are there any Little Libraries around where you live? There's one beside where I live and I've found a lot of really interesting books checking it semi regularly.