2nd what Moto said. Having also been in the dirtbike world, you really have to show sponsors why they should sponsor you, and not simply looking to gain from it. Sponsors want to know what they'll get out of it, and it needs to be really worth it. Might want to put some thought into it if you haven't yet. Decide what you can do for them.
From there, call the companies and inquire about sponsorship (and if it's even available from whomever you'd like to be sponsored by). You can email first but IMO calling is a more serious, direct step.