Since Outdoor lighting is visual, I think the first thing you need to do is get a website promoting your outdoor talents. Do an e-mail newsletter and drive people to your website. If you have to do postcards (personally I hate them) but your website on them. I good website is a great way to create instant credibility.
As far as what kind of population base one needs to make a living at outdoor lighting. I think it comes down to demographics. Do the high end prospects in your area know about outdoor lighting? do they think it in fasion to have outdoor lighting? Is there enough high end prospect in your area to keep you busy.
For me I don't think my area, is big enough to keep me busy. I have found a business partner to help me continue "Night Lighting FX." I have to start working for "the man" again. And between me and my partner "Night Lighting FX" will trickle along untill we get those right jobs that help propel us more recognition and more jobs.
My marketing resources are: networking and building strong relationships with the right professionals, tricking my website out with all my best work, thinking of ways to drive traffic to my website, doing quarterly e-mail newsletters, try to write columns on outdoor lighting, I am going to get involved with a well respected university in their "Landscape Architecture" program teaching artistic outdoor lighting.