Diapershop.com has it:
http://www.diapershop.com/index.php/...7/prevstart/0/
I've seen the bleached variety at Joanns... although I'm not sure I would trust the stuff hehe
If you're looking to save money though, you're better off just buying them -
www.wholefamilymarket.com has really good prices with volume discounts, etc.. you can buy a prefold for substantially less than it would cost you for the fabric. (at 2.50 per yard for the twill, considering you need over a yard to make 1 prefold, you're better off just buying it for less than 2.00 each)