Examples of pen pal letters can be found online, specifically on English-Room.com and Edutopia.com. Pen pals are often obtained through a student's school.
The website Edutopia also has a page devoted to pen pals. The "How to Begin" section has a "letter of introduction" link to information about what to include in a letter, as well as sample writing. In addition to examples, the page provides a great deal of background for teachers who are interested in having their classes explore pen pals. It provides reasons for including the project in the curriculum, as well as information about how to locate pen pals.