There are multiple dealers around for Monster Towers. Winnisquam Marine, Fays, MacCallums, NECC, Ossippee Lake Marina, etc are all listed.
http://asp.monstertower.com/usadealers.asp?State=NH
I would stick to someone that works on wakeboard boats as they will have a better understanding of the mounting options rather than a general mechanic with a drill and a hole saw.
One of our previous wakeboard boats was the 2007 Show Boat for Monster Towers so I can attest to the quality of their stuff. If you plan on doing lights, speaker pods, etc do it at the time of tower installation to save yourself some labor down the road.