I usually buy measuring stuff from Arc Euro Trade. Not the best quality, but everything I've bought from them has worked well, and calibrates fine when tested on various gauges.

I use the internal micrometers the most, but for most work a good set of digital verniers is good enough.
Note the word good, as some are horrendous. I had one cheap set where the internal jaws were over a mm different from the external jaws! Having a couple gauge rings soon lets you know just how accurate things are.