Heater Core part needed

I was wondering if anyone could tell me where I might be able to find a specific part I'm needing to get my '75 Stingray's original heater/air conditioner working again.

According to my mechanic, all I need a heater core 'inner case with doors'. But I seem to be having trouble finding such a part, and so is the mechanic. I can certainly find a heater core, but the inner case with the doors is the hard part.

Am I going to have to try a junk yard, or do they make such a thing anymore?

Thanks for any advice you can offer!

Mike