Here's several solo viola scores (free):
http://fac-simile.org/catalog/instruments/viola
http://www.viola-in-music.com/free_classical_music_download.html
Not overpriced:
http://www.cordamus.demon.co.uk/viola1.html
Amazing Grace - Viola:
http://members.tripod.com/~m_kimber/grace.tif
http://www.greatscores.com/p/song/songname/Amazing_Grace/instrument/Viola/linktext/Instrument:_Viola/artistgsn/1000286/sheetmusic/1007969/sheet/1
Book - 20 Sacred spirituals for viola & piano (including Amazing Grace, $20):
http://www.lastresortmusic.com/Shopping/product_info.php?cPath=13_17&products_id=200&osCsid=8eef630d0c705d9bf418a898609bf7df
"The parts of most interest are the fiddle introduction (a variation is repeated later after the first verse), and "Johnny's Solo" (the Fire on the Mountain part). Here're approximations of both":
Devil Went Down To Georgia (introduction)
Devil Went Down To Georgia (Fire On The Mountain Part)
(scroll 1/3 of the way down the page)
http://www.freesheetmusic.net/sheet_music_downloads.html