Children's Books

I've been wanting for quite some time to start collecting children's book. I don't have kids yet, but I would love to have a nice collection of books for them. It was interesting to learn in a class that a study showed a predictor of reading achievement was the number of books in the home (not just the amount of time parents read to their children). The only reason why I have not bought a whole bunch of children's books is the price! Especially the classic, really amazing children's book and kind of pricy. So...does anyone out there have any good suggestions on where to get children's books for cheap? They by all means do not need to be in new condition.