I do not know where you can find it wider than 60" but wanted to share that I recently made a large lap sized t-shirt quilt and considered piecing a sweatshirt type fabric backing for mine. I changed my mind when I had the weight of it all in my arms - it will certainly increase the weight of your quilt. Of course, that may work well for you but it wasn't going to work for my purposes.
Be sure to share pics!
Christine