Both of mine I got from US sellers on ebay.
The first one (used on my digitiser) was from the seller Deepgrove1, as he was the only one who actually answered my questions about using them on 24V inputs, however he has since stopped making and selling them.

The second one I got from Imsrv - http://www.ebaystores.co.uk/imsrv/Pr...sub=5194280011
I've got one of their adjustable ones, but I see they've stopped selling them, in preference for selling a pre-calibrated version of their fixed probe.

The calibration depends on what you plan on doing with the probe. If you're using it for digitising, then the calibration doesn't matter, however if you're using it for edge locating, then you really need it calibrated. The calibration can either be done in software, or mechanically.