They are hard to find now. I have a pair of them brand new I can sell you for $40 shipped. Otherwise, just look around on ebay or surplus stores to try to find some.

But don't buy these:

http://cgi.ebay.com/PAIR-2-NEW-JOHNS...QQcmdZViewItem

Those are the low rpm versions.

This is the high rpm version:

http://www.enigmaindustries.com/Johnson_Mini-EV.htm