I haven't had to replace any of the bearings- yet.
Most get damaged because guys like to over torque the mounting screws.
I throw away the little grub screws and use a 3mm machine screw that can be found at the local Home Depot.
I use a bunch of blue locktite too. Sometimes I have to use heat to get the motor out of the mount but I haven't had one loosen since I started using it.
I did buy some bearing oil.
Comes in a little bottle that has a needle on the tip.
I keep thinking about getting sealed bearings, they cost more than the motor does
But at that point I'd have a 30 dollar motor, instead of a six dollar motor.
But just maybe if it's a 30 dollar motor that ROCKS it would be worth it?