Do you know what the fixture name is? Look on the valve do you see any letters. M would be Moen, K would be Kohler, the rest they usually engrave an entire name on it.

Is there another bathroom in the house, if this is an original builder home fixture, most likely they used the same brand fixtures throughout the house.

Check Universal trim kits and see what multiple manufacturer valves they fit. You can get them at the Box store and bring back if they don't work.

Another way is to shut the water off and take the stem out, bring it with you to the store and try to match that way.

Good luck