You might be planning this already but an idea for you to make what's available wider or taller, copy a section in the center the length you need to extend. Cut the whole canvas in half and separate the two pieces and place the copied section in the middle. Clone the edges as needed.
When I do this kind of thing, it's usually to extend the background on a commercial portrait with one of those mottled backgrounds. I take smaller pieces to copy than half because the background is so uneven in color and in light.
Hope that helps if you end up licensing something already available.
If you are wanting to wait to see if anyone is willing to shoot, will you please provide us with a deadline - even if you don't have one - by editing your OP?
