Adventure Ski and Cycle in Sudbury (I'm not affiliated) has started renting fat bikes this winter for $40/day. They are Surly Pugsley bikes and everytime I've been in the store this winter (half-dozen or so), at least one of the bikes has been out on rental. The owner tells me that if there is one available, he'll take it home after work on Saturday and hit one of the local snowmobile trails or a local lake.