I've used these on a couple of bikes.

Blcycle Quill Stem 31.8mm

https://www.ebay.com/itm/J-L-NOS-Thr...s/153655631661

https://www.ebay.com/itm/351784121927

https://www.ebay.com/itm/253289874010

It looks like they're listed as 80mm and 90mm. I'm not sure if they are the same or different.



I've read theoretical complaints about the 2-bolt, but so far it has worked for me.



It looks like Nitto also has a welded steel 31.8 stem.

https://www.ebay.com/itm/Nitto-Ui-12...r/192887329028



Also a few MTB and adjustable stems.