Saw you had no responses...
Don't have a letter per se, but have information you may find useful.
First off, what donations are you seeking? Money for the PTO or for some other cause or product or gift certificates that you sell for benefiting PTO or your cause or generating PTO interest?
Good salesmen ask themselves questions such as what benefit can I give to the person I am selling to? What objections may they have for donating? Why should they 'buy' from me instead of someone else?
I'll give you some examples.
My dentist will donate to my kids' causes but he won't donate to those who do not use him as a dentist. The benefit here is he's building customer loyalty and he's realized that others who ask him for donations may have no intention of becoming his customer. Are you regular patrons of the businesses you approach? Would the business consider those in PTO and the school to be their target market for their product or services? will the donation get the business some free (and effective) advertisement to generate new loyal customers?
It would be great if they just donated for the good feeling of donating, but it doesn't work that way. Can you touch upon a stronger emotional response?
For example, my son is top seller for cub scout popcorn. He doesn't do this by selling popcorn. He does it by selling the donations to send popcorn to our military men and women overseas. He approaches businesses and individuals who already strongly support the military or are veterans themselves. The objection of 'I don't like popcorn' is overridden. The objection of "I can't afford to" is met, too, in that he'll accept as little as a dollar donation and combine them to make a larger popcorn for the troops donation whereas the cheapest popcorn is $10.
In this economy a business needs a benefit they can calculate financially. Can you solve a problem for them? Provide volunteer workers for something they need done? Give them a way to reach their target market?
You need to preconsider and address objections. For example, in collecting BoxTops forour school most parents see it as a hassle to collect those little rectangles. Give them a colorful envelope and sturdy fridge magnet so their home has a 'place' for BoxTops. I gave my school a school logo-decorated collections sheet that holds 50 BoxTops, so parents can view progress and an added benefit awarding kids who turned in 250 boxtops a cute bulldog pup (our mascot). Suddenly I had extended family working together to assure their kid got 250 whereas before the family only turned in 20 or so. Make it easy to donate to your cause.