Different applications call for different types of recommenders. It's most common, however for the app to call for your direct supervisor to fill out at least one recommendation. In your case your direct supervisor probably changes from transaction to transaction. My recommendation would be that you seek out one colleague for whom you've worked and has seen your work up close in personal that you can trust to operate in your best interest. Ask him/her to keep it quiet until your ready to publically come out with it. Most applicants I've worked with are able to identify at least one such colleauge.