Give Brian (Scooter on here) a call. He may have some stock.
Real Steel do them for about £130
Rodley replaced one (plus belt) and charged £300 https://www.two-sixties.co.uk/mgoc26...ight=altenator
Choice is yours
Rockauto in the States have them as well (Roughly £130 shipped for a standard 110A one, plus two-sixties get 5% discount at RockAuto) )