What on line shops did you try? When I'm looking for backgrounds, I'll often try the Fat Quarter Shop. Their photos of the fabrics are excellent, and they are easy to search. I usually put a bunch in my cart, and then one can view the thumbnails all together when viewing the cart, which helps in assessing how the fabrics compare to each other and what the distance effect will be.
Since it's such a major project, you might consider ordering half yards of some of your favorite possibilities, just to be sure. It is such a beautiful quilt!