Good luck. I found it in the FSM, but none of the online parts store appear to list it separately. Probably best to check with a dealer - hopefully you can buy it individually or it at least comes with the bleeder valve. Worst case and probably most likely is Nissan includes that part as part of the caliper assembly and you have to buy the entire thing for that one stupid cap (that's what it's called in the FSM).
Nissan likes to make cheap parts stupid expensive by pairing them with another part that is very expensive and/or labor intensive to replace. For example, Nissan says the $50 differential bushing is part of a $900 subframe member that must be replaced (add $1500 in labor) when the rubber seal inside the bushing bursts
