You're not looking for anything exotic, by your description. You could just go to the big, bad city Managua....but otherwise there are shops in Granada, and they can order from Managua if you don't find exactly what you want in stock. Probably the same in Leon, although I didn't need anything there.
Sounds like you're going north.....? If you don't like hanging in the big capital cities, parts and repairs are remarkably easy in Antigua. Don't know about Tegucigalpa, but spending time there finding out would be less appealing. Otherwise, lots is available throughout Costa Rica.
Hope that helps.