I've been searching for replacement non-A/C heater box nuts and have had no luck finding any used ones or other products that will work in their place.
I 3D modeled the nut and am getting prices from local 3D printers to make some. Is there any interest in these pieces? Looks like I can get a discount for a bulk order but they're still kind of pricey.
For two of these shipped anywhere in the continental US, it may be like $30.
Here's the model I created:

 

Have you tried this site? https://www.clipsandfasteners.com/de...SAAEgJPn_D_BwE

 

I just tried looking through that site. I found a Toyota clip that looks very similar, but it is much smaller than the heater box hole.
I'm sure something on that website would work, but I would need better diagrams and measurements of the pieces to know before ordering. I wish this place had a brick and mortar store so I could just go try and match up my part with the stuff they sell.

 

I don't know if they're all alike, but the local O'Reilly store here has an entire section devoted to small/odd hardware pieces in blister packs. Might be worth a try if there's one near you.