Make every effort to reestablish connections with college professors, and if necessary look for an appropriate substitute.
Call or write a professor – include a photo if that would help – and try to strike up a correspondence. Send that professor an old term paper or two, with the prof’s comments included. See whether, over time, you can jog the academic’s memory enough to get an enthusiastic and substantive letter.
What some applicants do is to take a graduate course or two and cultivate a relationship with each instructor. Attend office hours; ask for career guidance; assist in the instructor’s projects; go above and beyond. And if you have some prerequisites to get out of the way anyway, taking a course can kill two birds with one stone.
Sometimes a supervisor at work can attest to your quasi-academic abilities; for instance, a VP for whom you researched and wrote an extensive report. Think creatively. If you did have an academic recommender, what would s/he he write about? Is there someone else in your life or career who can attest to the same things?
Ask the b-schools for guidance if necessary.
If you are absolutely stymied in your quest for an academic reference, a call or E-mail to a business school can clarify whether their "requirement" of such a recommender is carved in stone – and you may get advice in terms of how previous applicants to that same school have coped with the problem.