Tachometers are available at most auto supply stores and department stores that sell auto parts. Dwell meters are useless these days since outboards haven't had point type ignitions for decades -- but there are a lot of them around. You wouldn't need a dwell meter for them anyway since setting the point gap with a feeler (thickness) gauge accomplishes the same purpose. Now it also depends on what kind of testing you are considering. Tachometers used for automobile style (battery ignition) ignition systems may not work on outboard magneto style ignition systems. If you buy an inductive timing light/tach that is triggered by a spark plug wire you would be able to use that on any engine. You need to define your plans in a little more detail.
I have a Merc-Tronic, which is what OMC dealer shops had back in the '60s. It has an inductive tach and a DC dwell meter. The dwell meter was used on I/Os, the tach part on everything.
