First.. If you don't have special insight into the industry, how do you know that they have a problem, and how do you know your solution solves the problem that needs solving the most?
That is a great question, in most cases. In my case it's a bit different. As I said in reply to Michael earlier:
I used to work with people that worked with ad agencies all the time. They always used to complain to me about this specific problem. It was something I had to deal with, A LOT.
I, personally, never dealt with the ad agencies directly. I have also spoken to a few people that work at various agencies and they have confirmed that this a problem. But those leads haven't worked out.
So it's not that I don't know the problem I am solving.
I have solved a problem that I know exists. Now I need to find the market that understands that particular problem and sell them my particular solution....if that makes sense.
In that case, don't worry about 100. Get your first user, through your friends who have ties within the agencies. See if your solution works for them, and then go from there.
Otherwise new orbit's suggestions are very good