This wiki contains a selection of sheet music in various alternative notation systems organized by composer. Below is a list of other sources of sheet music for particular alternative music notation systems.
Ambrose Piano Tabs
http://www.ambrosepianotabs.com/page/library
Clairnote Music Notation
http://clairnote.org/sheet-music-library/
Klavar-Foundation in the Netherlands (Klavarscribo)
http://www.klavarskribo.nl/
Klavar Music Foundation of Great Britain (Klavarscribo)
http://www.klavarmusic.org/
(The Chroma Institute website previously sold music for Albert Brennink’s A-B Chromatic Notation, but the site is no longer available.)