Several Manufacturers use this Sandia 6gallon body . If you Google Cleanstorm 6gallon you will see that steambrite fit a 300psi to their version ( also needs the additional plumbing for pressure adjustment ). There is a thread on the steambrite forum where someone asks if they can fit a 500psi but Steambrite claim there is not enough room for the bigger pumptec and twin vacs .
Other options might be the 150psi diaphragm pumps ... or aquatec make a 200psi version , or perhaps the 250psi rotary vane used in the Prochem Endeavor .