You can get anime sheet music for viola here: http://www.animeonstrings.org/violasheets.htm
Since the viola is not a principle instrument in Evanescence's music, it is very rare that you will come across a viola sheet for their songs.
I'm sorry to say, that I don't know. Musescore.com does sheet music, but there aren't many Viola songs, and none by Cloud Cult. Sorry! I just find it cool that, well I play the Viola too, and that song^^ (When Water Comes To Life) is what inspired me to play it! http://www.insanity-comes-naturally.tumblr.com/ Message me/send me an ask? :)
Sheet music plus 8 Notes Free Scores
Animekeys
Yes
There are many website available about viola music. I include one of them here.
For free sheet music, go to the 8notes website and search 'Taylor Swift' or the title of the song you want.
imslp.org Best site ever!
In a music store
how to play amazing grace on a viola. provide a transcribed music sheet.
how to play amazing grace on a viola. provide a transcribed music sheet.
If you Google it, a link to go to 8notes.com shows up, that has a variety of free viola sheet music from bach to beethoven