I was in the same situation as you when my ceramic impeller shaft for my eheim canister filter broke while I was cleaning it (WHY in the world are they ceramic in the first place?! Geez... you'd think that something like stainless steel would be better... ugh).
Anyway, for the time being, I just used a BBQ scewer made out of bamboo, and I would just shave it down to size until the impeller itself would spin easily around it (keep in mind, that after a couple days/weeks, that the bamboo might swell up when it absorbs water, so don't forget to factor that in).
Or, use some other material that's lying around, that will temporarily work.