For Business Letters:
Instant Business Letter Kit contains more than 125 professionally written full-length downloadable letter templates for real-life business situations. It also includes over 200 pages of letter writing tips, techniques and information. You can instantly download the templates (MS-Word) and revise them as you please.
Instant Business Letter Kit
In addition to the above format for "personal letters", you may also be interested in some of the other typical letter formats, depicted elsewhere on this site, that are usually used in pure business situations:
Other Letter Formats