I tend to buy fabric I fall in love with. I love to shop my local quilt store, with a coupon or a sale but have also been known to walk in and buy a bunch of coordinating fabrics full price because I loved them sooooo much!
i buy a lot of fabric at garage sales too. Many good deals are found there and I have quite a stash of 30s fabrics from sales.
I recently bought about 25-30 yards at Hobby Lobby because there were gobs of coordinating fabrics marked down to $4 and less per yard.