I am not sure where you are located, or to what type of agent you want to market to. The answers to these two fundamental questions will guide you to the best answer. For example if you want to market to Agents in your area in the commercial field, LoopNet, (and I don't work for LoopNet), has many ways you can market to Agents. You really could start there.
If you want to market to the residential field, I would suggest getting a list from your State Real Estate authority. While this list is not likely to have email addresses, you can begin a process of developing your list by contacting Agents by mail or by telephone.
Lastly, I would bet there are services to be found with email lists in your area that can be purchased. Check your Internet Explorer.
Good luck.... Good hunting!!!!
Rob Baird, CA RE Lic. #544165 (One of the oldest, active licenses in CA)
Oct 5, 2010