American Marine (Pinpoint) and Milwaukee are common.
Probes are replaceable and last only so long so when a Canreef vendor (
http://www.aqua-digital.com/ ) was offering theirs at about 1/2 the cost of others, I got a couple. Seems to hold calibration. When I pulled up their link, see they have cheaper monitors now as well.
Make sure you get the test fluids.