Anyone know where to find good deals on children's books?

I'm constantly on the hunt for good deals on children's books. My kids are growing like weeds, and their reading interests are changing so fast that it feels like we need new books every other week! While our local library is fantastic, I also love having books at home that we can keep and reread. Retail prices can be pretty steep though, especially for hardcover picture books. I actually recently found some really affordable Kids book boxes that made building our home library way easier without spending a fortune. The books often come out to around $1 each, they're usually in great condition, and there are no duplicates in the boxes, which is a huge plus. Does anyone else have favorite places or online sources for finding quality children's books at really low prices?