Two things.
1. Call JoAnns customer service (make sure you have the sku # of your fabric, They can do a nationwide search for your fabric. They'll give you the phone # of the stores (if they have any) and you can call them and check to see if it's in stock. If not, try #2.
2. Here is the link to John Flynn's diagonal pieced backing.
http://www.flynnquilt.com/workshop/FreeLessons/ Scroll down to Diagonal Pieced backing and print the instructions. Do the math and see if it will work for you. I haven't tried it yet, but am going to try it tomorrow for a charity comfort wrap.
Good luck. That said, been there, done that. More than once, I'm afraid.