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
Clairnote Music Notation
Klavar-Foundation in the Netherlands (Klavarscribo)
Klavar Music Foundation of Great Britain (Klavarscribo)
(The Chroma Institute website previously sold music for Albert Brennink’s A-B Chromatic Notation, but the site is no longer available.)