Don't have one here but just wanted to wish you luck on your search and please update us if you find one and your son's reaction! (Given Herry's more recent obscurity that's wonderful that he's your son's favorite!)
When i was a kid i had all the commercially released hand puppets (there were lots of them in the 70's) but when Herry came out and i got him, it was a particular joy since he was one of my favorites and even though he was used much more back then than he is now, it always seemed he wasn't as popular/heavy featured as Big Bird/Oscar/E&B/Cookie/Grover/Count so when i finally had a Herry puppet i was overjoyed and had him with me everywhere for a while!