I can comment on the two ways I've been found, which is mostly applicable for finding grad student consultants.
(1) People have searched the proceedings of conferences and emailed me off of those papers (2) LinkedIn. Join either the machine learning or (3) Graduate school directories. Me and several people from my old department got the same email. It was clear whoever sent it, sent the same request to everyone who listed "Machine learning" as an area of concentration
Best, Aria