Oh no, I hope they don't start cropping up on eBay.
In my experience the Koma is almost indispensable when following Mejiro, and merely a luxury following Tenjyou, but diluting the slurry and continuing to break down either of the middle nagura slurries with more laps seems to accomplish the same function as a koma, before jumping to the tomonagura.
Especially the tenjyou, these slurries can be pushed pretty far in terms of breaking down and bridging to the next nagura.
At least with my experience the tomonagura can, with repeated refreshings (like 4 or 5+), accomplish quite a lot on its own. It just takes more time. It should be noted this method definitely does not have time on its side, it is more about achieving zen/tao timelessness than churning out shave ready blades all day long, IMO. Most I can do is 2 in a day, for now.
So don't fret over the koma my dear friends. These stones are amiable friends and great teachers if you choose this method.