I'd try your local AS dealer, Inland RV or Out of Doors Mart if you want an exact replacement. Otherwise, any shut off valve you can find at an RV parts store or hardware store that's the right diameter inlet and outlet will work just fine.
Thanks Chris, I searched a bit on-line for these things before I initially posted but no luck and I just found out this morning that our only local AS dealer is no longer an AS dealer. So now the closest one is 350 miles away...bummer! I'll give Inland RV a try and keep searching a bit more.
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