Examples:
http://www.canadavisa.com/canada-immigration-discussion-board/all-kind-of-affidavits-and-sponsor-letters-to-study-abroad-here-t206637.0.html
https://www.sais-jhu.edu/sites/default/files/affidavit_of_support.pdf
http://www.stgregorys.edu/sites/default/files/page-attachments/INT%20Affidavit%20of%20Support.pdf
http://www.bbk.ac.uk/prospective/downloads/ExampleLetterParent.pdf