Most of the gaskets are a dense foam rubber. I've used this foam to make tail light gaskets for one of my cars and it seems to be the same stuff used in the heater boxes. 3mm works out to just under 1/8".
All the rest you'll have to wing as you go. Of the heater boxes I've had apart, there's nothing in them that you can't duplicate or replace with a little common sense. Take it apart and figure out the thickness of the foam as some may be 1/16" or 3/32".
The foam... Yea, I know, it says "cosplay" in the description, but if I compare it to what DMT sells, it's the same stuff. Foam:
Amazon.com