I have every book I ever owned from my childhood. I adored books even then.
I also flea market and yard sale often. You'd be surprised the number of people that have a book of books for sale at $1 each who will give you the whole box for $15 or $20 just to get rid of it. I've gotten beautiful condition books this way.
Another good one is the school book orders for kids if you get them. Lots of deals if you have a good look through those every month.