I don't know what is the most receptive way to market to agents because not every agent likes the same things, however I would probably recommend that you first try taking flyers for your service to the broker offices and ask to have them distributed to the agents. This wil lprobably be the most cost effective way to get in front of a lot of agents.
You can also try sending an e-mail flyer through one of the companies like Iannex (http://www.iannex.net/) that are used by agents and thoise trying to market to agents via e-mail. Some agents have opted out of services like these, but it's also a cost effective way to reach a good number.
One suggestion - Make sure your flyer and service come across as professional. Testimonials mean a lot - testimonials we can call and speak with mean even more. We get inundated daily with people wanting to sell us things or have us recommend their services to our clients. Show why your service should be considered above the others.