Remove the broken bearing using a screwdriver or a drill. Most models have several smaller screws that hold the bearing or its plate in place. Remove both of these, then the actual bearing.
Take the bearing to a hardware store. Find the same size and model or something as close as possible.
Screw the bearing into place. It should fit snugly in the already existing hole with the bearing plate under or on top of it, depending on the model.
Reinsert the two security screws into the plate. Tighten each one down.
Test the rocker's gliding ability. Adjust the bearing if you notice a problem.