What can I put on my belt buckles and buttons because of a nickel metal allergy?

Nail polish. Apply clear nail polish over the metal. It will last longer and is usually effective.
Wax. Avoidance is the best thing, but if that is your favorite belt, try using paraffin wax on it, and it may create a barrier to help prevent problems. Keep in mind that it will have to be reapplied, like the wax on your car.