If you look on Amazon under BMX Quill Stem there are four listed. Two note that they are for 7/8" bars but the other two may be for 1" bars. Ugly but they look strong and are a lot cheaper than a new fork to get a bike back on the road while looking for a more appropriate stem.

These are all modern design removable faceplate stems and look like there is enough meat in them so they could be reamed out for larger diameter bars.

Also a Amazon search for .833" Stem shows a number of more traditional looking steel stems available, most made by Wald.

A search for 21.1mm stem shows some more available items, mostly Pyramid brand.

thanks for this data!

Similar results can be found on ebay, searching for:

21.1* stem
.83* stem

I'd have a hard time putting one of the sturdy, modular BMX stems on my Continental.

I'd love to find a stem that had good forward reach, without dropping down... more like a Nitto dirt drop.

This is the closest I've found, on ebay, from seller aaabicycle (no affiliation). My Continental is black, so this might not be the end of the world:

Sunlite makes 21.1 high rise stems. I swapped one at my LBS to outfit my Schwinn Super Sports. The stock stem was killing my back. The new one gives me a far more comfortable ride.