Truck equipment dealer
I would try a local truck parts distributor for an air solenoid valve. I bought mine (only one they had) out of a bargain bin for $10 at Layton Truck Supply. Has 1/8th inch ports. It is made by Sculley but had trouble finding a web page for them.
You may need to resort to the internet for finding them. Type in air solenoid and you will get a list of suppliers. Make sure they are 12 Volt!