Summarize:

Warren Buffett is a trustee of the Gates Foundation and has pledged the majority of his wealth to the organization. Communications sent here may have a small chance of reaching him. The mailing address for the foundation is PO Box 23350, Seattle, WA 98102.  Note that the Gates Foundation does not give grants to individuals and rarely accepts unsolicited proposals. If the foundation is interested in providing a grant, representatives usually contact the recipient directly.  If you are connected to a nonprofit organization, you have a much higher chance of getting a response from the foundation. Don’t address the letter to the Gates Foundation, but to Buffett personally. This may avoid the letter getting lost in the thousands of other letters the foundation receives. Spell out clearly why you’re contacting Buffett and what you need from him. The organization may pass along your letter.  State in the letter that you’d like it forwarded to Warren Buffett. The Gates Foundation is much more likely to take letters seriously if they align with the organization’s mission. It largely focuses on improving health and education in the world, so if you have a cause that lines up with this mission, your chances of a response increase. office. Increase your chances of someone forwarding your letter to Warren Buffett by sending the same letter to both of its US-based locations. The Washington D.C. address is 1300 I (Eye) St. NW, Suite 200 E, Washington, DC, 20005. The foundation also has several international offices. It's unlikely letters sent here will reach Buffett, but if you'd like to try, there is a full list at https://www.gatesfoundation.org/who-we-are/general-information/contact-us. It is an extremely long shot that any letters you send  to the Gates Foundation will reach Warren Buffett. He doesn’t work there directly, so this letter would have to pass several channels before he ever saw it. Understand that this means you will probably never receive a response.
Find the contact information for the Bill and Melinda Gates Foundation. Write a letter directly to Warren Buffett. Send the same letter to the foundation’s Washington D.C. Accept that you probably won’t receive a response.