Does anyone know if there is currently a stem available for vintage Schwinns (21.1mm) that accepts 26.0mm handlebars?
NO, there are none. That stem diameter became obsolete long before 26.0 (derived from Italian racing stems and bars) became the de facto standard for road bars. Before that, 1" (25.4 mm) was the most common clamp size.
But... you could probably either mill down the 22.2 mm quill section of an existing alloy stem with a 26 mm. clamp... or perhaps easier still, widen a handlebar opening from 25.4 mm. (= 1") to allow a 26 mm. handlebar. A small rotary sanding drum on a Dremel tool works pretty well.
If your bike has a steel stem, you may as well forget about trying to "modify" it.
With alloy stems, I've had little difficulty sanding down the shafts of some standard stems to fit into 22.0 mm French steering tubes. Just need a good long sanding block - and a lot of patience.
Something like this inexpensive hand sander found at paint stores (designed for sanding sections of drywall) measures around 8-3/4" x 3-1/4" (225 x 80 mm.). The stainless steel spring clamps at each end quickly and firmly grip the ends of a simple strip of sand paper - which you can cut from a large standard sheet. This one is made of rigid molded plastic with a thin rubber base which backs against the sandpaper.
Or, if you have a belt sander which you can clamp down in a vise (upside down) you can just slowly spin the stem around the rotating abrasive and save a LOT of time.