We have the Vaisala unit on our offshore installation. Just gone 10 years continuous operation before first incidence of u/s. I know from experience that the pilots rely heavily on it.
Is is part of a Fugro Geos system. See them at
http://www.geos.com/
As mentioned before another vendor is Muir Matheson.
I would expect to pay 'Oilfield' prices for both of these companies. Not sure if you can get a 'cheaper' version for onshore.
Our system was originally installed by InstallOcean, who were then bought out by Fugro. I see they still have a web site at
www.installocean.co.uk/ so you could try them to see have done a CHC on it.