One option is to cut your fabric diagonally and then shift the two pieces along the diagonal so that you get the width you need.
Here's a picture that may explain it better than words can. The first shape (a square) shows what you want to end up with. The second shape shows what cuts you need to make in the yardage. (The exact numbers for the 8 / 42 divide depend on the exact (usable) width of your fabric and your desired seam allowance.)
Caveat: I have never tried this method myself. I have read of the technique in multiple places as a way to eke out additional width for a backing when you don't have much fabric to spare. Advantages of the method are that all the fabric remains in the original orientation (so no having stripes running the "wrong" way on the added piece) and that the seam is not going to end up being on a fold line when/if the quilt is stored.