I got a chance to look your ride up in ASIST today, and the bad news is the dealer wasn't lying. Both versions of the Q60 have "mat" type OCS systems built into the lower seat cushion and N/I does not sell any service parts, the entire lower frame has to be replaced. Before you do that, I'd suggest having someone yank the seat bolts and carefully inspect the seat harness for chafing or pulling, many OCS issues turn out to be wire failures from broken harness restraints or incorrect harness routing.
That said, if the seat is bad, a good junkyard seat is your best option. Exact prices for a new frame depend on your option package and color, but all versions list for around $6500~$7500. You can save about $2K off list through an Infiniti-authorized online vendor like InfinitiPartsDeal, which would still come with a factory parts warranty if installed by a dealer. The good news (if there is any) is that it shouldn't be a big labor charge. The frame comes upholstered, so changing it should only be a 2~3 hour exercise.