You need to find out what type of bearing it is - usually either caged or sealed.
With caged you can just replace the bearings.
If the bearings are sealed you need to find another bearing exactly the same size OD (outside diameter) ID (1 1/8th steerer or 1" steerer) and depth (v.imp ) and also make sure that both bearings are the same.
As well as these you also need to ascertain the angle of the inner and outer edges - usually printed on side of old bearing and either 36x45 or 45x45.