We had lots of extra books to start with (duplicates or unread in ages) but I filled in by visiting my library's discard rack. Our town library is VERY small so the turnover is quite impressive - I've often found fantastic, new, award-winning, and generally unharmed books here. Children's books are 5 for $1. - wow.
Kids and parents seemed to enjoy it - and the above picture is what was left over (lucky me, right?)
Give this idea a try at your next party for little ones - it sure beats a bag of sugary treats and junky toys. The best part? Putting a book in a kid's hands.