I bought a tach amplifier at http://www.ips-parts.com/
See attached picture.
A new one cost about $75 - unless you select a dealer to make them rich, in which case I bet they won't sell it for under $100 ?
A good junk yard might have one but you should ask them for a guarantee that it works, these get flaky (the tach stops working or is intermittent). One solution to the flaky problem is to stuff something under the cap so the pins get forced into the socket tighter, I found that solution to be temporary.
Anyhow the socket is what you will probably better find at a wrecking yard, a dealer is likely to have a problem getting one, or if they can its likely to be fairly steep. You will also need a bracket to mount the socket, I made one out of scrap aluminum, mounted on the fenderwell. Sorry I don't have a picture of my DIY setup handy - maybe tomorrow?
Dieseldiehard