The first is http://www.apologyletters.net/, which offers over three dozen different apology letter templates. Templates include apologies for various behaviors (being late, not attending an event, shoplifting (!!), etc), apologies to various people (boyfriends, girlfriends, judge (!!), etc.) and apology letters for businesses. The letters come in a plain text format with template information in bold, but you can also download them in Google Doc format.
Assuming you don’t need a letter of apology to the court, you might find Cut Files more interesting. Cut Files, at http://cutfiles.net/, offers 150 SVG (Scalable Vector Graphics) files for download and use with SVG-compatible personal die-cutting machines that cut paper into shapes. (I didn’t know anything about personal die-cutting machines, but I got a good overview of them here.)