You could try Koh's Camera (
sales@kohscamera.com) - they have ceased camera repair operations, but I have the impression (gathered from someone else's impression) that they may have some parts around. Maybe they let their parts go and can let you know to whom they let.
I just bought (last week) an AE prism finder G from KEH, and earlier in the year talked to them about repairing my defective finder. The person to whom I spoke mentioned that they had some parts. That person gave me incorrect information about the possibility of a GS-1 repair, so I wouldn't get my hopes up that KEH actually does have parts, but it might be worth a phone call.
I had a brief email exchange with Tamron Canada a year or so back, but that came to nothing for the GS-1. FYI, they did indicate that they would have a shot at repairing a PG lens, but would make no guarantee that they could actually fix it, or that they would have the necessary parts to fix - but they would have to charge for whatever time they put into the repair, successful or not.
Is that something that could be 3-d printed? Floppy-like? I could imagine casting one if you have a part to make a mold from.
cheers
Tom