Are you talking the float? If so, I know Fluidmaster style valves, have a replaceable rubber gasket/bladder, that can get gunk buildup, or eat up by chemical's thrown in the tank (like bleach tablets). The bulb style, I know you used to be able to get replacement bulbs and the gasket that goes to the tank for it (the gasket where it goes through the tank to the supply line, should be available for all styles).
I am not sure about the small, fill pro style valves (normally used in small toilets).
Also the flappers are replaceable, if the water is leaking into the bowl from the tank.
Shutoff valves, are either a ball valve (replace, not fixable, but lasts longer and quicker shutoff), or the traditional style (with a rubber gasket).