Try the following web site:
www.vibrantperformace.com (sorry I am not the best at the link thing).
Look in their car accesories (not truck), towards the end of their on-line catalog I think you will find the MAF adapter for Nissan.
I am 80% sure they are all the same but verify through them.
The next problem is that they do not sell through the internet so somewhere in their web site they can direct you to a local distributer for them.
I think the price is around $16 for an aluminum adapter.
As far as the warrenty goes (has been said a million times) it only screws up the warrenty if the dealer can prove that the mod you made actually caused the failure.
Good Luck