That's a tough one. I've been pinning any free e-book sites that I come across on my library's Pinterest site, but they really just seem to be picking ones off the kindle top 100 free list. There really is a lot of self-published stuff that needs a good editing, at the very least!
Let's see, there are always the classics that are out of copyright. Gutenberg.org is working on those, and there are lots already on Amazon. You can lend books to others. Try to find friends and create a lending circle to share the books you already own. If you have Prime, you can borrow one Prime listed book per month, I believe.
I tend to just scan the top 100 free list once per day or so, to see what is new that has popped up. I do use overdrive thru the library, too.