On EBay, a seller by the name 'BGGameBoy' regularly dismantles Rivarossi locomotives to sell as parts. Seek out item #295380829008 which is exactly what your looking for and is available right now from him. Reasonably priced as well.
PMR
I read somewhere that the Berkshire used the same drivers as the 0-8-0, also the Mallett. I've owned all three and AHM/Rivarossi had a habit of using just a few drive systems for all of their steam locos. The Berkshire drivers are way undersized compared to actual scale as a result.
Anyway, the 0-8-0 in particular, is very plentiful on the used market and often sold for parts only. It could be a good source.
I have a couple of older (1970's) HO scale Rivarossi steamers that I had to put in storage for a number of years with all of my train collection. When I finally got to take everything out again, the driving wheels on both Rivarossi steamers had become so brittle that the wheel centers tore out of the wheels when I tried to run these locos. No other brand steamers I owned exhibited similar problems. I considered purchasing new drivers from Greenway but the cost was more than I had paid for the locos. Both locos are too large for my current layout, so they remain in storage with there ruined drivers.
Hornblower
Is it the gear that's broken off the axle, or the drivers?If it's the gear, you might be able to get a replacement gear, along with a suitable gearbox, from North West Short Line. The gear for the axle will need to fit properly, but I'd guess that NWSL would likely know what's needed.
Wayne
I'm looking for a set od drivers for a Rivarossi berkshire. I have one that the driver with the gear on the axle has broken off the axle. I've tried to find a set of drivers, but so far no luck. Does anyone know of a source for these?