Nuosu Yi is spoken by a couple million people in SW China and is written using several hundred characters representing individual syllables. Further info can be found on the Babelstone Yi Page, and links to several useful resources are available here.
Both OS X and iOS come with fonts for reading/writing Unicode Yi.
The logical way to type Nuosu Yi is with an input method like those used for pinyin Chinese. You can obtain such an IM via this link.