Thrifting can be a wonderful way to obtain fabric. I mostly look for yardage but have also harvested garments and have done projects with Hawaiian and Plaid fabrics, and one with "ugly" ties. Make sure to check out the linen department for UFOs and aprons can be an amazing source of vintage fabrics. Linens is usually where yardage/scraps are kept but they can be in crafts area too. My feeling is once it hits the thrift stores, it's value is whatever it is wanted for... if I am wearing someone's "Too Good for Everyday Wear" sparkly Christmas sweater while I'm doing dishes, well -- it cost me $5 and I'm happy. If I cut apart shirts or even wedding dresses, it's getting used. I haven't actually done the wedding dress thing myself -- but I have helped friends salvage what they wanted on several.
Goodwill and the other large chains typically have "color" tags for their items, they come in on one color, stay a month and then that color goes on sale. In the Seattle area, Tag day started on Thursday with a slight discount (like 10%), half off by Sunday and often there would be a Monday morning special whatever it was would be $0.99-$5.00 -- even if it was a couch or sewing machine or whatever!
In large areas, you might also have a "Buy the Pound" or final stop. Caution, wear gloves in those piles. Is amazing how dirty "clean clothes" can be.
Not all thrifting is equal. Large cities have large selections, smaller towns not so much. Even within the Seattle area I had Goodwills that rarely had what I was looking for and not worth my time to check on a usual basis. But when we are out and about traveling we will often stop in small towns and I will be very happy with something I find.