Peter, I'm thinking about doing the same as a sideline in the off season. Business cards are easy to get made and pretty darned cheap for a box of them. They are great for handing out to landowners when you're out shaking hands.
In this area there are hundreds of miles of drainage ditches and this Spring when I was out asking for permission to trap them, I discovered an opportunity. There is a "Drainage Ditch Society" that landowners pay a significant tax to. I haven't met one landowner that is happy with the assistance that the society gives in the way of upkeep of the system (ie beaver dams and erosion of the banks due to muskrats). As far as I can tell, the society doesn't have anyone to trap the animals and they merely destroy the dams.
In other places in the county, the County Works department will put out live traps for beaver but then they'll take the trap in after catching only one.
If you can find out if there's a riparian committee, or something similar, you might have somewhere that is already in place to gather complaints and pass them along to you. When I'm ready (next year?), I intend to talk to the society and landowners to let them know that I'm available for rodent control. It probably wouldn't hurt for landowners to be contacting the society and asking them why they haven't sent out "Trapper Dave" to take care of the problem.
It'll take me a year to prepare for this venture, maybe two, but I think that there's a niche for it around here. For now, I've been meeting with landowners and handing out business cards. Next year I plan on attending the society's bi-annual meeting and introducing myself to those in attendance and providing the committee with a proposal.
If you don't have anything like what I described in your area, I would suggest contacting places like golf courses, etc that could potentially need your services. I'd definitely be contacting the county so they have the opportunity to contract out for your services.
Good luck with your venture.