Source : Webster's Revised Unabridged Dictionary (1913)

Sustainer \Sus*tain"er\, n.
   One who, or that which, sustains. --Waterland.

Source : WordNet®

sustainer
     n : someone who upholds or maintains; "firm upholders of
         tradition"; "they are sustainers of the idea of
         democracy" [syn: {upholder}, {maintainer}]