> What's the best place to look for one?

The original Westy tank was designed to take an Auto Stop valve, which had a safety feature that prevented the tank from being overfilled without user intervention.  Unfortunately Auto Stop Corporation went out of business years ago and the valve is no longer made.  A conventional fill valve can be fitted instead, and is generally available at RV dealerships or propane specialists. You would also need a manual bleed valve to screw into the bleeder connection next to the fill valve, and of course make sure to use it when filling. (If you already have a separate valve there, someone probably converted it already.)  As I understand it, technically a shop is not supposed to replace an Auto Stop valve with a conventional one because it defeats a safety feature that the tank was designed to have, and overfilling a tank can be extremely dangerous. (Our new replacement tanks are designed to accept a conventional valve, and that is what they come with.)  That being said, it is a relatively common modification.  

Ron Salmon
The Bus Depot, Inc.
www.busdepot.com