The Collaborative International Dictionary of English v.0.48:
Sustainer \Sus*tain"er\, n. One who, or that which, sustains. --Waterland. [1913 Webster]WordNet (r) 3.0 (2006):
sustainer n 1: someone who upholds or maintains; "firm upholders of tradition"; "they are sustainers of the idea of democracy" [syn: upholder, maintainer, sustainer]