In reply to zammi:
Have you tried your local wall? They will most likely have large sizes amongst their hire shoes and perhaps could sell you a pair.
Red Chilli Session's go up to a 15 but I think they only get sold direct to climbing walls.
Evolve do a pair but I can't remember the model name.
If you get desperate/decide to spend a bit more I would try 5.10 as some of their range seems to be available in larger sizes more often than other brands.