Regarding Mapbox, see https://www.mapbox.com/help/add-custom-fonts/
I assume you have access to that font if you are registered at Mapbox.
These fonts are copyrighted by Microsoft. According to https://www.microsoft.com/typography/fonts/font.aspx?FMID=1081, Arial Unicode MS Regular is shipped with many Microsoft products, but not freely available elsewhere.
You can read here how to integrate any font in Mapnik:
https://github.com/mapnik/mapnik/wiki/UsingCustomFonts
http://weait.com/content/add-fonts-your-openstreetmap-server