Most alternator remanufacturers/reconditioners should be able to sell you one Dave.

It's the main unit which regulates the current/voltage that the alternator outputs. Unfortunately they come in alsorts of various different sizes and designs so they are not easy to describe over the net.

I am not sure but if they are a lucas alternator then they will be a black plastic body with an aluminium cap held on by 3 x 5.5mm bolts and is obviously a seperate unit to the alternator when you look at it.