So, I took a quick look around and came up with this list of GaA wafer manufacturers in the USA...a few interesting names and locations, particularly M/A-COM, although it seems they only produce 150mm wafers.
Others claim they are "high volume production" facilities, but not sure exactly what that means.
http://www.thomasnet.com/products/gallium-arsenide-gaas-32640450-1.html