Quilt stores are an awesome source for cute woven prints, but your wallet will know about it. Luckily diapers don't need a lot. There is a FMBG coop group on yahoo, plus the sewing supply coop is good for fabrics and elastic. I have also seen wahms buying preorders on osds. Sewzanne's sells the ever-diaper-popular PRR prints. And there is always ebay. Joann's just got in a bunch of knit prints.
Someone, I can't remember who, also sells diaper cuts for good prices. Janine's PULs if that is what you want, and there are several diaper wahms who also sell fabric. I've seen fabric for sale at Valor Kids, FMBG, and I think Darling Diapers.
I would give you actual links but I am naking!

I am sure I forgot a bunch too. OK, here is one link:
http://www.cookiepants.com/sew%20what.htm This page has a ton of resources. HTH!