See if there are any Annie's Book Stops around you. Also check with the library - sometimes they have book sales of donated items. Also go to yard sales.
If you're really industrious, see if the local library and the supermarket will allow you to post flyers, and organize a book exchange for gently-read children's books. People can either just donate, or they can take a book home for every one they bring in. If you belong to a church or synagogue, they may let you use the space for free as a way of promoting literacy and barter in a down economy. Sometimes a local Newcomers Club has play groups that can be a source of networking and items-to-trade.
Good luck!