Grizzly Mr. Joe Piccirilli says a 260 comb will fit a Koch. I can get one from Hohner for about $12.00. Might be worth just to see if it will fit.
Yes, it will. They're essentially the same harmonica. The only differences I know of are that the Koch doesn't have windsavers, the "The Chromonica" (aka 260) does, and the covers have different words on them. The reedplates may or may not be slightly different, reflecting the different tunings (Richter v. solo), but the combs are the same.
That can't be said of the Seydel Standard or the Hohner Educator. The Seydel will likely be the wrong size, because it's likely the mouthplate holes are spaced differently from any other main maker. And both of their combs are made to accommodate reedplates without windsavers, which means the chambers most likely won't line up with a Koch's plates.
Those plastic combs have only two reeds per chamber, and twice as many chambers as a Koch as a result. The Koch has four reeds per chamber, just like its fraternal twin, the 260—and every other slide chromatic harmonica Hohner currently makes other than the Educator.
Good luck in any case. Joe P's offer of a Plexiglas comb would be the answer to your prayers, and you'd end up with a comb that will never dry out or change dimension. IMO, forty bucks is cheap for that comb.