For a year now, I've collected and donated used children/young adult books to teachers/schools in my city of Philadelphia. I recently cleaned out my bookcases and garage at my mother's house and found 200 books that were hidden from me. Usually around the summer, I give them to those who need them for their classroom libraries or even school libraries so for the school year, they have reading material for the kids. Because a lot of schools in Philadelphia have outdated books or not enough, this is more of a passion and not a chore :) I'm running low on how to collect the books since I've cleaned out every house in the family and need help oh how people can donate more. I tend to stay away from library sales, but I'm constantly in thrift stores buying with for $1.00 with extra money I have on the side. Is there any other way I can collect the books for my project without spending money? Seems like I've done it all, but I might be missing something. PM me with your answer and thank you for your help.

Join your local freecycle (online).  People post what they no longer want and you can respond that you want it.  Also there is a free section on craigslist.  I see people giving away books free all the time, especially kids books.

One of the gals in the I WISH thread was putting some K-6 books for an Indian school that burnt down. I PMed her so I could donate something too! :)

I second the idea of freecycle and craigslist. Also, you might want to check out homeschoolers' curriculum swap meets. You can usually find lots of books on all kinds of subjects, and if you go later in the day you may be able to get them very cheaply.

Get local scout troops or religion classes at churches to do a book drive for you.  We do this all the time. Parents appreciate getting their kids involved in volunteer wrk without it costing them money out of pocket!