get the gear kit from http://www.odometergears.com/bmw.html. you'll have to check to see if you have a vdo or motometer instruments, and it might help if you pull your instruments and the tach to verify that you're ordering the correct gears.
i've been through this on both BMWs that I've bought - the gears WILL break. even if you somehow find instruments with working original gears, they won't last long, and it'll throw off your car's odometer reading. buying new gears is the only reasonable way to go, even if it's a bit pricey.