How can I fix a shower mixer ?

Got a chrome bath tap set with lever in the middle which moves in a small arc one way operates the shower other way runs the taps into the bath. It wasn't working that well but a guys been out to fix and has broken the inner diverter valve. It's pushes the mixer valve back and forth to switch between the shower and bath. Any ideas where to get a spare ? Or do I need to buy a new tap?