For years now, I buy most of my fabric at thrift stores, estate sales, etc. I will also buy new fabric but I try to limit that to specific purposes, not just buying it for the joy... but sometimes joy wins. Many of us quilters have that moment when we see the pattern or the fabric or whatever it is that triggers us into the next project (or maybe the project after that). So sometimes I can go look for the fabric that does what I want, other times I look at the fabric and see what does it want to do.
Mostly I make scrap/charm quilts so I do need a wide variety of fabric, but I'm really starting to realize I don't need the yardage I have so I have been giving away some fabric recently. I've also been organizing my stash better but it's time to make up some projects/cut some fabric/make a new mess out of the order it's taken so long to almost achieve
The thing is about thrift stores is you can only get what you get, not more and not less. So maybe I do have to buy an entire bag of scraps just for that one piece that is calling me, or with my postcard blocks where all I wanted was a tiny little square but had to buy the entire piece of yardage to get it. Most importantly, you have to go regularly and if you see something you want, or think you might want, you need to grab it!
I've had several projects lately that required large collections of fabric. I do try and give away scraps from such things and will hopefully finish that postcard quilt soon and declare a substantial stack as surplus and time to set free.