Where do I get Sun Tach box batteries?

I have an old 9k rpm tach with a nos sender box, but the box runs on some goofy batteries. Is there somebody that makes a conversion or is there a place that has the batteries?

Looks like this, only this is some pic I got off google.

 

use AA flashlight batteries, they don't make the mercury batteries anymore

 

Wasn't there a company by the name of Williams, that did a kit for the tachos?

I hav a local electronics repair place make a voltage regulator and wire it into the tach sender - whatever comes out of the generator - 10-14 volts is converted to 2.7 volts, as required by the sender.

It has 2 1.35 volt Mercury batteries connected in series, so requires 2.7 volts to power it.

 

It's called Williamsons Instrument Service. They arent cheap but you get what you pay for. I have heard you can use the lithium camera batteries in the Sun sending units.