Yes, Mamiya and NPC Land Polaroid backs work great in the RB67 with Fuji pack film. B&H has the cheapest prices I've seen and they still have the FP-100C, FP-100B (while it lasts, discontinued, though Fuji USA says they have so much they're worried they'll still have some after their 11/2011 and 12/2012 expiry dates so no real rush) and FP-3000B in stock.
Fuji USA may start dumping the FP-100B before it is stale dated so the prices could be good soon. Very sad as it is wonderful film but obviously we don't shoot enough of it. The FP-3000B works too, even outdoors so we'll still have color and B&W pack film options even when the 100B runs out but the 100B is better.