just use normal hops, but hop to a very low IBU
using aged hops isnt as important when making pLambic because you are adding the desired bugs, whereas in traditional lambic making you wanted to limit the unwanted critters and large amounts of hops did this
Ive made many pLambics with run of the mill hops, I do like lower AA ones though, but thats more of personal preference than a technical one
Im planning on doing a spontaneous ferment soon myself, Im just waiting till it cools off a bit, and I will use a very large amount of aged hops, hopefully this will help inhibit the nasty critters that will ruin the beer