The Purdue University website and Google Docs both offer samples of formal business letters. In addition, a downloadable template of a business letter is available for Word 2007 and later from Vertex42.com.
A formal business letter should contain a header that includes the addresses of the sender and recipient, as well as the composition date. An appropriate salutation should follow this header. The body of the letter should be single-spaced and each paragraph should be justified to the left. Close the letter with both a signature and a typewritten name. After the closing, list any documents or forms that were included with the letter.