Where can I find a 1995 GMC Rear Heater Diverter Assembly

My 95 full size GMC conversion van that has a rear heater diverter valve. It's about a 2' long pipe with a three way, vacuum operated actuator on
the top. It went bad several years ago, so I bypassed the rear heat. Meanwhile, I tried for a long time to find a replacement. The dealer says it's not factory part (it looks very GM), and the upfitter says it's not theirs. In fact, the conversion upfitter (Gladiator, currently out of business) sent me one of theirs, which is a typical plastic, vacuum operated valve you'd expect from aftermarket. I sure would like to find the correct replacement. Anyone with a van of that era that has experience the same problem? Any ideas?
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