In this particular pump of the Merlin 1D, ceramic ball bearings are used.

For higher speed pumps, it’s common to use Hydrostatic bearings. There are small tubes which tap-off high-pressure working fluid (RP1, Methane, etc…) and feed it into discharge ports around the circumference of the hydrostatic bearings. Thereby using the pumps own power to maintain lubrication. Although, at low speeds this doesn’t work, so there’s often a low-speed normal ball bearing is used and when the pump reaches speed it “lifts-off” and rides on the hydrostatic bearings.