Spring Book Sales
May and June are big months for public library book sales. College libraries, too.
Sometimes, the books are close to giveaways.
It's a great place to take reading-age children. Give them $5 each to spend. But they have to return whatever they don't spend on books. This is a great investment. It will make them book lovers early.
You can use Google to search for your region's sales. Or use this: http://www.booksalefinder.com
You never have to feel guilty about buying used books, cheap.