Hmmm...
If you search for something like:
451 Presta Tube
It will weed out a lot of the junk you'll be finding when you try to search by 20x1.
Hmmm... I've been trying to stock most of the common tube sizes that I use, but for some reason I just haven't been bumping into those.
I did, however, discover that one can take your standard 700c tube (new?), and carefully insert it into the tire, and fold it back on itself.
It will get you back on the road in a pinch, and may well be good enough for permanent use, especially considering the 451 tubes seem to be about 3x the cost of 700c tubes.