BigKev is right, check the box on the firewall, it will give you a clue to what brand you need. Check with Kanter or Max Merritt as well. 6V positive ground.
If you do find a used one and the condition is suspect, run it to a local alternator repair shop, they usually rebuild generators as well. If you need, I have a local one you can deal with in St Paul, MN, they rebuilt mine.