I spent a while looking for a Conair Rotron or EBMPAPST distibuter that would deal with selling only a couple of motorized impellers...no luck, The one I liked the best was a Comair
DD402112B1R. @ 4' in Dia it is physically smaller than what I came up with and moves less CFM but thats not a bad thing in this case. Rated fom 6 to 14v dc, at 12v it moves 0 CFM @ 1.1"wc and 97 CFM @ 0" wc...it should move plenty of air @ 6v. If anyone knows of a source let me know.