Good idea, but then follow up with a stage II remote round, where you just start finding agencies through the country & beyond. People in your city you should use the more personal touch, but these folks you'll prolly just want to email.
DO visit each of their sites & see what they actually sell/market
DO make 6-8 version of your cover letter to fit different niches, and tweak each one to be a bit more personal before sending out
DO get a decent way to track them like they are customers (ie, a CRM or some good tool) so you remember to stay in touch.
Don't send anything that reads like a mass-mailed spam! Something as simple as "I see you are all in Boulder. My Dad went to UC" may sound a little corny but they need to see they aren't getting a form letter, and they need a way to remember you out of the crowd.
It is a lot of work, but over time it pays off.