Mics are pretty simple and bulletproof. You don't (and can't) calibrate them. Mechanical ones are just a screw and a vernier scale - they get their accuracy from how accurately they were made at the factory, not from being calibrated after they are made.
For basic 1"/25mm mics -
The only thing that may get out of adjustment is the zero point. If you set the vernier to read zero when the mics are completely closed, and the anvil and spindle aren't visibly trashed, you'll get factory accuracy out of even a very old tool. You don't need any precision standard or reference to zero them.
For mics larger than 1" or 25mm -
They're the same as basic 1" mics except you use a standard to zero them.
Mics larger than 25mm are always sold new with a suitable standard and a wrench for zeroing them. (The wrenches are almost universal and interchangeable between common brands - one wrench will fit most mics of most sizes of most brands.)
I wouldn't normally buy used mics larger than 1" that didn't come with a standard. For 1-2" mics you need a 1" standard to zero them, for 2-3" mics you need a 2" standard, etc. It's true that you can use other references to zero mics if you don't have a suitable standard, but it's convenient to have the standard close at hand in your tool box for a quick check.
I prefer old-school mechanical mics - If a digital tool goes bad, it's probably not practical to repair it. Unless they're seriously abused, non-electronic ones basically last several lifetimes without any loss of accuracy or need for repair. I like the basic Mitutoyo 102 series ones, but also have some from other manufacturers. The Mits are nicely finished and easy to read, even with my less than perfect eyesight. I keep thinking that I might buy some digital ones, but I don't do production work or make huge numbers of measurements, so I've never bothered to spend the cash for the slight extra convenience of a digitial display. If I were a professional machinist, I'd probably have a full set of digitial ones. I've bought most of mine either 'NOS' or used. Nothing too old - not because of accuracy concerns, but because the markings on really old ones generally aren't as crisp, contrasty, and easy to read as newer ones.
Even if you buy digital, it's probably still worthwhile to have a few mechanical ones around - you know they'll stay accurate more or less forever, and you'll never need to change a battery or worry about a failing display.
The name has changed, to Scherr-Tumico.