I never set out to build a stash on purpose. It happened slowly over many years - like 40. LOL I used to make most of my clothes and also for my DS when he was young. Always 100% cotton. There were scraps left and I saved them. I bought a little extra for each quilt I made and added the scraps to my collection. I went through a phase of making only watercolor quilts and bought many flowered FQ's. Again, saved the scraps. There were several years of making baby and child quilts and saving those scraps. When my mom could no longer sew due to Alzheimers, I received 20 boxes of fabric because I'm the only daughter of 4 who sews. About 1/3 of that was cotton.
A couple of times a year, I'll go to all the quilt shops near me to check out their bargains/sales. If something really jumps ups and grabs me, I'll buy without having a project in mind. I'm always on the lookout for neutrals, paisleys, reds and soft light golden yellow. I also find cotton fabric at resale shops, yard sales, church sales, etc.
Now, when I start a project, I start shopping in my closet and then buy to fill in as needed.