Small and medium-size businesses make up most of the companies in the U.S.--an unrealistically huge target for almost any entrepreneur.
Narrow your focus based on the most important qualifying characteristics of your best prospects. Look for publications, websites or other media that these prospects read for information on the type of services you market. You want your message to be in the right context.
In other words, you don't simply want to reach merchants anywhere they may go . . . you want to reach them when they're in the right frame of mind to be receptive to your message.
Best of luck!