Good marketing (not just advertising) sources would be local real estate investor clubs. I know you already do this; I heard a presentation in Alexandria, Virginia, a year or so ago. Some investors have become skittish about going it alone, and would feel more comfortable as a franchisee of a company such as yours.
Recognizing, though, that your franchise is somewhat expensive from an investor standpoint, you might look at other demographics. (I'm sure you already have.) People who are retiring from the military, for instance. They're still relatively young and energetic, and may be looking for another challenge. Same with people retiring as policemen/women.
As Sue suggests, making yourself known to franchise consultants is an excellent idea.
You might also find some formerly successful mortgage brokers who've been hurt, even lost their jobs, from the sub-prime meltdown. They already know an aspect of your business.
Hope that helps.