Look here: http://www.drumcorpsplanet.com/forums/index.php/forum/292-dcp-sponsorboard-2011-season/
Perhaps more importantly, contact everyone you know (or can think of) who has a job or a business. Compose a compelling story about what you are trying to do, and why, and be ready to sell yourself. Be specific in your request. Example: My corps will perform 30 times this summer. Will you commit to $10 per performance? Be creative. There are many ways to get this done.
People with money are out there, and will help a kid who is trying to do something significant. But you have to ask. Nobody is going to voluntarily give you money. If you cop out and convince yourself that no one will help, that you aren't a salesman, that it can't be done...well, you're right. And you probably shouldn't be marching drum corps. But if you are willing to suck it up and proudly ask for support, you'll get it.
Good luck.
Dave