I bought a couple from Taiwan Joe. His prices are great, and he ships super quick. I have bought lots of stuff from him and everytime he's right on the money. Normally ships next day.
Mine did not come with instructions, but I have a copy.
Check out the 13k analog tachs on Ebay, there's a pic of one on my Raven chopper thread. It was cheap and I presume it will work with 6V, the specs and wiring is a puzzle, there's info on forums about them though, lol!