So not going to write one....
But I'll give you some ideas. These are general points:
You'll have to write it from their perspective. Meaning what is your group about and HOW is it going to benefit them and their company/business (e.g. are you going to give free advertisement, add their name to your banners, send your group emails or in what way are you going to benefit them... not just 2A).
Remember it's a business deal for the most part. You got to give them something.... - you don't typically get something for nothing...
Keep it short. You can always give them more info in as supplements to your letter but don't bog your letter down with too many details. Keep it one page with key points and what you are looking for (hats, money, guns or whatever).
Also list what accomplishments your organization has done, if any...
If you are a 501(c) or other make sure you list your info. I guess some can always "write it off."
But basics:
1) What's your group and who are you in that group
2) What your intending to do
3) What you want $$$$$
4) what your group can do for the business (what's in it for them)
5) Do take pictures and/or news clippings and send them to the business - be a thank you card everyone signs or a roster of folks who attended to pictures of folks standing in front of whatever they gave and their logo.
Good luck! Hope this helps.