Schwalbe makes 'em in 20".
Near as I can tell, there are four ways to deal with studded snows:
- Put 'em on and leave 'em on.
- Swap the tires as desired.
- Use two wheelsets, swapping as desired.
- Use two bikes.
There is no correct choice, only whatever works best for you.
EDIT: This applies to tires with carbide studs. They hold up well against dry pavement. Plain steel studs are probably best swapped ASAP after a melt. /EDIT
My first winter I planned on "Put 'em on and leave 'em on". In 700C, they're much heavier than road tires, and I quickly adopted "Swap the tires as desired".
My second winter, I used two wheelsets, swapping as desired. The other advantage there was that I could use a lower-range cassette to help compensate for the added pedaling resistance. The road wheelset has a 12-23 and the winter wheelset has a 16-27.
Currently in my third winter, I put fenders on my fair-weather bike too. I have lots of choices now.