36" = 1 yard
27" = 3/4 yard
18" = half yard
9" = quarter yard
I've NEVER been to any quilt shop or fabric store where you could not ask for inches instead of yardage. In other words, if you need 20 inches, they should be able to either cut 20 inches, or tell you the closest fraction, which in this case you are correct, it would be 5/8. In other words, don't be afraid to say "I need 20 inches, what's the closest to that you can cut?"