Check out sponsorhouse.com, there are a lot of grass roots companies on there. Also, start with you local shop and see if they have a team. That will get you started and if you ride well, they might be able to put you in touch with bigger sponsors.
That being said, don't expect to get a bunch of free stuff for nothing. Being worth sponsoring is more than just riding well. Be prepared to help at at demo's, go to comps, and push your product. Otherwise, don't bother, you'll be a drag on the industry. When approaching a company or rep, have a video to show your skills and be prepared to tell them how you can help their company out. Finally, if you do get sponsored, don't be a stuck-up jerk to everyone else. Help the kid who's learning 3's or sliding his first rail.
Other than that, good luck...