Called tap revivers/tap restorers.
Like has been said above, you may find the splines of a new reviver are a different size or the stem is longer, so you’d be best of buying a reviver with a tap head included. Plumbase are the cheapest near me
They’re actually called tap valves. you can buy a tap reviver kit which is the valve plus the tap head.
Previous posts are correct in saying the splines may be different, there are literally thousands of different styles and sizes of valve. Take the valve and your tap head to a proper plumber’s merchant (not B&Q, Homebase etc.) and they’ll find you a match. You should also make sure the screw fits into the top of the new valve.