That's a very pretty quilt. Do you need a border to make it bigger, or do you just think every quilt should have a border? The reason I ask is that I think this quilt would look great without a border. And don't forget that you will need fabric for the binding.
If you do need to make it bigger, you could wait until you get the fabric for the back. Order enough so that you can do a thin inner border with the backing fabric (1" wide), and then do piano keys with whatever fabrics you have left over from the front, mixing in a few keys from the backing fabric.