I just had to return some fabric to an online fabric store -- I won't give the name because you'd all recognize it instantly. I've always used this place when I wanted to be absolutely sure the fabric was of good quality. I was deeply disappointed when I ordered 4 yards of a fabric, at over $11 a yard on sale, and you could see straight through it. So I returned it without comment. This was obviously a buying or supply error.
I do shop the thrift stores, yard sales and estate sales. You'd be amazed at what I've found there: High quality fabrics people just wanted to get rid of cheaply for one reason or another. Of course you have to be both picky and lucky, but at least you're there to pick up the fabric, look at it, feel it, check the print (is it on the grain?), check for flaws or damage, etc., etc. When you go on line, you have to depend on the reputation of the site, and past experiences. I will, of course, use this place I sent the fabric back to again. I've had too many good experiences there.
So I shop the full spectrum: from thrift stores to on line to regular Quilt shops. It just depends. I shop sales, ends of bolts, coupons, every savings I can get. And every once in a while, I buy something full price because I just have to have it. You know what I mean.
One of the things I've also done is to establish a relationship with 3 different fabric sellers on eBay. I used them often enough, and gave them great feedback, and they noticed me. (I was testing them: Did I like both the product and the service?) Then we had email "conversations". I'm very up front about what I like, especially in terms of quality. I let them know that I would be very unhappy with poor quality material, and delighted if they'd let me know if something special came in. So that is now happening. I'll see a fabric I like, e-mail them about it, and they'll truthfully let me know if it's something I'd "like" --- i.e. if it fits my requirements. That prevents me from buying poor quality material on eBay. They will also e-mail me if "something special" comes in, and they've come to know the kinds of prints, colors and quality I like. In return, I shop their stores, and continue to provide great feedback. They're helping me out by pointing me in the right direction in terms of their wares, the same as any good clerk would do if I were in a store, and in return, I am rewarding them by buying from their stores. It works. I think they also know that if they started leading me towards poor quality fabric, I'd stop buying from them, and I would. This is a great approach because it works, and saves me from wasting money on fabric I don't want. You can steal this approach from me if you want!
Happy Quilting!