If you're talking about a piston turbocharger tach, why would you want one?
Generally tachs for turbine equipment use tach generators which are magnetic, and generate an electrical signal. The gauges are self-powered using this signal. The tach generator is a small generator, or in some cases only a signal generator, producing a value that is displayed at the instrument as an RPM, or more typically, percent of RPM.
With respect to a piston engine using a turbocharging unit, manifold pressure provides most of the information you need. The speed of the turbo unit isn't particularly important, or relevant to your actions from the cockpit.
Besides, that little hummer is turning faster than you really want to know, as a pilot. It would probably just keep you worrying. Ensure that your fuel flows are proper for your power settings, that your manifold pressures are where they should be relative to a particular throttle setting and expected fuel flow, and that your lower end manifold pressure is where it should be at idle (throttle plate closed)...higher indicates induction leaks.
Where turbine units have a dedicated RPM observation function, an accessory pad is provided somewhere on the unit to drive the tach generator. This signal may be used only for cockpit instrumentation, or may also provide input for other functions, such as overspeed and fuel governing functions. Often this drive is coupled with another engine function, such as another accessory like the oil pump. In other words, whatever's turning the tach generator is also often turning something else. Knowing what is coupled to what may be an important clue in the event that you have a malfunction indication in the cockpit. For example, a tach loss may be associated with shaft failure when coupled to an oil pump; lose the shaft and you lose the oil and the tach signal. Understanding your systems may give you important knowledge and insight into not only what you have, but the appropriate course of action under any given set of circumstances (and even the appropriate checklist to use...).
Magnetic pickups are also used for RPM functions, but not generally for heated and stressed components such as a turbocharger.