I assume you're going with a church or some sort of charity group? If so, talk with them about doing some sort of fundraiser. Begging for money on the internet really won't get you far, but having some sort of community event probably will. Be creative. Back when I went to church, there was a group that would frequently sell dinners after mass. It usually worked out quite well for them.
Somewhat OT: What is it that you actually do on these "missions?" Mostly charity work? Or is it proselytizing? The only real context I have for "mission work" are historical, where the missions would basically imprison natives and force them to convert. I know this isn't how things work nowadays, so please help me understand.
