Our king size mattress is 72" X 80", and has 12" radius corners at the foot end. We just had a new mattress custom made, all latex foam with different density layers, with the same dimensions, and we just use standard king size sheets. They aren't a perfect fit, but work well enough for us. One company that I found, and I'm sure there are others that custom make bed linens, is
http://customlinensdirect.com/