Office for Mac 2011. The Calibri font family is distributed and licensed by Microsoft within Windows 7, 8, Office Suites, and other products. Independent of a Microsoft product, it can be purchased.
If the Open XML File Format Converter (2011) from Microsoft can still install on your machine, that is a free source of Calibri, and other MS Office Fonts. There is no assurance it will install on Mavericks. If one has Pacifist, only the fonts can be extracted and dropped into the local ~/Library/Fonts folder. (Cough.)