You have to email Rex to order the solenoid valve: advice@rexgrigg.com
The one you referenced is a Milwaukee valve. My regulator is a Milwaukee all in one unit with that Milwaukee valve on it. The valve failed to close after 2 months in use, which is why I purchased one from Rex. The one from Rex is ruggedly made, looks like it would be reliable, and has been for over a month now. And, it is cheaper than the Milwaukee one you referenced.