plywood for doublers

Phil, I just accessed Sig Manufacturing's website. Sig is currently relocating to a more modern facility.. and will resume full operation for 2023. They're only accepting orders for "in-stock" items.. but the plywood you're looking for.. appears to be in  stock.

Sig weblink

I got some 1/32 ply from Hobby Lobby.

The Thin Mark

No idea where to get 1/32". I can attest to using whatever plywood I had in hand for a couple of planes. But then, I typically don't build for the long haul.

I do have a stash of recycled fruit/orange case plywood for when things get even tighter in our little hobby world. One of my boys' first planes had a 1/8" firewall because it was what was on hand. Ifni came down to it, I'd use 1/8" on one side and call it done. You'll laugh all the more having one in the air that isn't "precious."

Don't be too shy or too picky unless you are building to keep it.