To get a scrappy look, I would use fat quarters cut in random widths to get the most variation. Yes, it might not be the most efficient use of the fabric, but the quilt is designed for scraps!! There's going to be a trade off if you use yardage.
Here's her string block primer:
http://quiltville.blogspot.com/2005/...ng-primer.html
If you use a foundation (paper or muslin) and mark your 4.5" block, you can position your fabric for the least waste when you trim it up. Your strings don't really need to be a single length of fabric. Just gather up the left overs, separate them by widths and sew them into a long strip.
Personally, if you have any thrift shops near you, I would look for men's shirts in beige/cream/white. There's a lot of fabric in a man's dress shirt. Or ask your family & friends if they have any shirts they are ready to donate. Cotton bed linens are also an idea, but if you want to get lots of variety, look for pillowcases, not sheets.