My stash, which is not very large, is made up of fabrics I have found on sale, mostly REALLY GOOD sales. I have several grouped together to be used together (like kits) and others just grouped by colors. I love making scrappy quilts, but, since I am still fairly new to quilting, I don't have a lot of actual scraps. I now have enough of a stash to have plenty of variety in my scrap quilts.
Usually, when either I or my mother buy fabric, we buy for each other as well. Generally, if one of us runs out of a specific fabric, she can get more from the other. I had to get some from her a few months ago to complete a quilt. Just a couple weeks later, I was making her a bargello and needed more of the fabric I wanted to use for the border, so I shopped from my stash. I had gone back to store where the fabrics were purchased to see if I could get more, but they were out on both occasions.
If I wasn't into scrappy quilts as much, I would probably just buy for specific projects, too.