I know this is a old post but I wanted to give you my experience in dealing with big companies towards sponsoring events.
I hold monthly fishing tournaments to raise money for MS. The best time to contact Big Companies is December through February. After that most of there money for event sponsorship is used up.
It is better to contact them by mail, not email. It is best if you send a packet to include a detailed letter of what you are doing and what you are trying to accomplish. They will want a flyer for the event. Good contact information.
With it being so close to the event I would focus on companies not associated with fishing. I use a lot of gas stations, grocery stores, restaurants, and things to that effect. They are usually always willing to donate gift cards.
If you have any more questions feel free to email me
fishing4charity@yahoo.com