Hard to say who has a "parts barn" in their back yard. Posting like this usually sparks a response.
If you get no hits, I recommend Harry Sheets,
http://www.midwestwillys.com/ , as he has a nice inventory of original parts.
Also, shop some of the military vendors. I'm guessing the 3A and M38 may share the same seat frames.
The 3A and 3B seats should be the same. I think the 2A seats are a bit shorter.