First, check out the new
FIRST Fundraising Toolkit.
Second, call the companies rather than send letters, let them know you will be showing up at their place of business a week after the call between X and Y time. Mention that you will drop off a packet with more information about your team (A sponsorship packet) and would like to give a presentation if they have time. Try to set up a meeting if you can, rather than a drop off information and leave.
My team used that method earlier this year and you can hear about our successes in the webinars published with the Fundraising Toolkit.