As to what to use, how about shim stock? Or buy them from ISKY or whoever has them these days. Machinists can order them for you, they aren't hard to find.
More important question is WHY you would do this? The longer valve stem doesn't do you any good unless you're going to shave the head to boost the compression. The common setup is to shave .080, then use the N42 valves because they are .080 longer, and shim the cam towers up. The shims return the cam towers to the right height with relation to the timing chain, and the longer valves put the rockers in the right location with relation to the cam and lifters.
Your original post seemed like you were trying to get the biggest valves into the head that you could. The P90 valve is the biggest Nissan put in, and it has the same size head as the N42, so there is no advantage to swapping out one for the other in terms of flow capacity.