I usually only buy fabric for certain projects, but, it's only true in a sense.
If I see a fabric I love, I look through books and get ideas of what I want to make. I love the planning stages of a quilt. I will buy enough of each that I could use them for most any pattern I would like to. Then I have enough of all the fabrics for a quilt.
My stash has come mostly from swaps and extras from other quilts. I've also been given lots of fabric. I don't usually buy just a FQ here or there or just a yd. I'm big on 3 or more yds of each fabric so I know I will have enough.
I have several large drawers full of fabric w/ pattern for specific projects.
Lynette