My last project has an Airpower gear set, that forced me to do some research on that company, here is what I found out, they are no longer in bussines, DW is so far the only place in the US where we can get some parts, there is this page showing all kinds if wheels but, I don't know how real that is.

AirPower Model Accessories

I have an overseas friend investigating where to get replacement tires and parts but that is a shot in the dark.

A bit of info on wheels, the shaft, bearings and brakes for the 2 3/4" are the same for the 3" wheels, you can use them as mains or nose with the solid replacement tires, the 2 3/4" wheel will expand to 3" when inflated, the brakes are very strong at 100 psi so, 50 psi is plenty pressure to slow down or to stop the aircraft, one more thing, DW has the 2 1/2" solid tires, replace the air filled with those, only $9 per set.

That's all I know.