Instead of buying half a yard, why not ask for one-third of a yard. That's 12 inches instead of 18. If you can't get nine usable inches out of that, I'd complain to the store manager about how sloppy the employees are on fabric cutting.
I guess I'm lucky. In most of the places I buy fabric in my area, the cuts are usually made one or two inches beyond the measurement I ask for, so I have a little bit of room to get my full amount.