maybe check that the wiring isn't touching metal somewhere. Had the same thing in my van and was paranoid my batteries were about to die (just before driving over the Pyrenees!), with the battery light coming on whenever i turned a corner... turned out the plastic covering had worn on part of the wiring and was occasionally touching metalwork.
if you need a new alternator you could always take van to a spares shop and show them what sort it is, should be able to pick up a replacement.