Ummmm. That's a very good question!! One we are all asking all the time!!
My favorite online/catalog place is Connecting Threads. They have nice fabric for a good price. You can buy tons of fabrics on eBay. You can sign up at JoAnn and Hancock for their sale flyers, and there are always coupons in those. You can hit garage sales. There are lots of places you can get fabric cheaper than at the local quilt shop.