RcMonster has them listed on that link, as: "1/8" bore Mod1 pinions"
Looks like they are out of stock right now, but usually things aren't out of stock for very long.
There is also a "bore reducer" that you can use to fit a 5mm gear onto a standard 1/8" motor shaft. here:
So if you got that, then you can use any Mod1 5mm shaft gear with your 1/8" shaft motor.