Where to get hub-centric ring?

I ordered some new wheels (stay tuned!) but their counterbore is 73.1mm. I know our car is 67.1mm, so I'm looking for a set of 73.1-67.1 adapter rings. I know the one site, 1010tires sells them and they are in Canada (preferred for me) but they do not have 73.1-67.1 listed.

So can anyone recommend me a place to get these rings?

Also, what the advantages or disadvantages to using either plastic or aluminum rings?