Dunno about previous versions, but series 3 (82-89) and series 4 (90-94) spiders don't have a voltmeter, but a dummy light in the speedo or dash cluster.
That aside, Autometer offers a voltmeter that is a pretty close match for the Jaeger gauges and it comes with some rubber booty thingies that you put over the bulb to change the color. (red, green, blue)
With one of the Alfa angle bezels (if you actually need it) it blends in quite nicely.
One can be seen in my spider (PO destroyed the radio hole with a sawzall, so I got creative) in the lower right position
in this attachment.