Make the blocks, then take a couple with you to audition fabrics for sashing and/or borders. I do that pretty much all the time. Then a lot of times, after I complete the top, I go audition fabrics for the backing, if I had not decided that up front. Sometimes I start with the backing, then audition frabric for the front LOL. I don't have a "stash" and purchase fabric as I make each quilt.