I also have a Mitutoyo micrometer that I got about 10 years ago. It is admittedly a luxury item (I think it was around $150 back then), and I don't use it very much, but that's mostly because I haven't gotten around yet to the project for which it was intended. (That doesn't happen to anyone else, right?)
In the Japanese definitives of the late-19th and early- to mid-20th century, there are a variety of paper types and thicknesses (not just thick and thin, but 4 or 5 different thicknesses). Because I wanted to do a specialized study of these definitive series, I got the micrometer.
I had discussed this device with other stamp people before getting one, because the risk is, as has been pointed out here, with some measuring devices you could potentially damage the stamp by torquing the device too tightly on the stamp. (Kind of like using rough-surfaced tongs.) This micrometer is so gentle that it's not a problem. I would think that any of the Mitutoyo micrometers that are electronic would have that same characteristic, but it would be smart to take a test drive with some stamps that don't matter much.
As for where to get one, I went to a big industrial plumbing supply store (that mostly caters to the trade, but is also open to the public), where they had quite a few to test out in the showroom. I also had the help of salesman there. And I brought several stamps with me.
The model I have is the Mitutoyo Quick-Mini PK-0505.
And just a little detail, in case you're interested: the brand is pronounced as if it were spelled "Mitsutoyo." So that "tu" syllable is pronouned "tsu". The company uses a form of Romanization that was more common in the past than it is these days. ("Nippon-shiki" instead of "Hepburn". More than you ever wanted to know here:
http://en.wikipedia.org/wiki/Hepburn_romanization ,
here:
http://en.wikipedia.org/wiki/Nihon-shiki ,
and here:
http://en.wikipedia.org/wiki/Kunrei-shiki !)
Cheers!
-- Dave