I am kind of in the same situation. I want to make a string quilt and have very little scraps. I have decided to buy small widths of fabric to use in my quilt. I went to Hancock Fabric and had them cut 6 inch wide pieces off of 10 different calico's. It was only $5.80 because it was on sale. I will keep doing this with sale fabric til I get enough for my string quilt. I want my string quilt to be made with calico's. For some reason, I am obsessed with calico's and 1930's reproduction fabrics.
If I were going to make a jelly roll quilt and didn't have a stash, I would probably buy a jelly roll. Lots of times you can find them on sale at Missouri Star Quilt Co. (online) for great prices. And if you trust Ebay, you can find some there.