mast halyard sheaves

I need to replace halyard sheaves at mast base. My trip to local boat store didn't find any compatable size. Does anyone know the brand/manufacture for replacement shives in the mast base or specs for replacement sheaves?

I found out the bolt on the mast base does not have any nut on the end and the old sheive can be taken out easily. I replaced it with a 2" X 1/2" metal sheive with a bearing in the center. The fit was perfect. I would recomend that you use a breaker bar and loosen the sheive bolts at least once a year to clean and grease the bearings.