I spent probably my last 20 years primarily collecting and using fabric from thrift stores and estate sales. Being in the Seattle area, there were craft sections where I could buy bags of small pieces of fabric, individual large pieces, and sometimes even bolts. Originally I was just looking for 36" wide vintage fabric but so much lovely and much more recent fabric would tempt me. Also look in the linens for party aprons often made (and never or rarely used) with full widths of 36" vintage fabric, for UFOs or other lovely items of linen, damask, cotton and embroidery.
Sure, I couldn't go out and find exactly what I wanted, when I wanted it, and in the yardage needed but I was able to find quality fabrics by every designer/brand. You name it, I've gotten it with a thrift store tag on it! Hand dyes to batiks, Balson Erlanger to Makower UK, lots of 80s era Cranston VIP... You learn how to burn test fabric to determine content -- if you aren't sure it's probably rayon/blend and I have decided I am ok to use if I can't tell. Wool and silk smell, polys edges turn hard and sometimes burn fast and hot (not suitable for children's pajamas!) or sometimes cast an oily smoke. I usually don't use what I call the "quality" blends, but lots of broadcloth or similar good for garments that is 10-20% poly and mostly cotton.
When we traveled we would stop by large thrift shops, usually Goodwill, an we still found things. Here in Central Illinois we still stop in small towns and I still quite often find things. I know if I still took scraps/small pieces I found a large bag of batik scraps I would have been very happy to work with, but I don't want to deal with the storage of the scraps.
In the Seattle area (and most large urban areas) they get so much stuff donated they can't keep it all. All the stores had some sort of dating tag and things were only kept for a month and then they had to get trashed/further recycled. In Seattle on Thursdays the prices would start their first drop, and us "pro" shoppers went out and got the things we were most interested in. Over the weekend the prices would drop more and then typically on Sunday (if they were open) or Monday, anything left a month old was $0.99... furniture, sewing machines, shirts, etc. Thursday was my day, I would head out in the morning with $20 and usually home within 2-4 hours (and up to 6-8 stores) and mostly I came back with some good stuff, other times I came back with my money intact but that was less often.
Yes, some people "donate" their garbage. You'd be amazed by the amount of stuff that doesn't even get to the sales floor. The bag of Hand Dyed fabrics I snatched up were not "fixed" that is, the dye ran like thieves in the night... But my car was in the shop and I had a Bonnie Hunter project to do, so instead of just putting that bag in the "textile recycle" (at the garbage transfer site, is what you did with old pillows and such) I ended up doing a bunch of boiling fabric and laundry.
You do have to control yourself when you look at the value of things... can be real easy to suddenly you have 5 very nice blenders or whatever and slide off the list from thrifter to hoarder. I had a list, top of the list was Fabric (preferably vintage, also quilting), then my Dishes (Yorktowne by Pfaltzgraff), then I had a list of picture frame sizes I was maybe looking for (hint: never pay full price for a picture frame ever again!), Vinyl albums I was looking for (I had a lot of friendly competition among the vinyl collectors and could direct someone to a store currently with the old 78s. Never did find my holy grail -- Weird Al, Peter and the Wolf in vinyl, but I've found a lot of amazing stuff.