Connecting Threads or Thousand Bolts on line would be your best bet. Good prices. You can buy wide backing fabric
90-108" wide or 45" fabric and cut to the length of your quilt, then cut the second piece in half and put on each side. So you would have 3 pieces to the back, not a seam in the middle.
Hello from Texas. I used to work at Ft. Hood, TX, an Army base.