The least expensive approach is to contact the real estate department directly for franchise companies that you think might be interested. This would be based on the fact that you've observed that other locations they select are like yours and that they are not represented currently in your area. The real estate department should be able to tell you fairly quickly if it has an interest in your site. This approach takes some effort but can be very effective at a low cost.
The next approach would be to contact the major commercial real estate brokers in your area to make sure they know of the site availability and your interest. Most large franchise companies have relationships with one or more large commercial brokers in each major market charging them with finding potential locations to run by their real estate department. You need to make sure your site is on the radar screen of each of these brokerages.
The final approach I would suggest is to advertise in media that are directed as much as possible to the audience you're trying to reach. In your case, since you're interested in food franchises, it might be in a publication like Restaurant Newsespecially if it has regional or market-specific advertising options available.
Jeff Elgin has almost 20 years of experience franchising, both as a franchisee and a senior franchise company executive. He's currently the CEO of FranChoice Inc., a company that provides free consulting to consumers looking for a franchise that best meets their needs.