The official Porsche spacers are spendy - about $250. You can get them much cheaper from aftermarket suppliers. H&R seem popular, I'll attach link to a merchant selling them (this is just for reference I don't have any experience with this merchant).
I got mine for free with my wheels from wheeldynamics.com, you could inquire with them on buying them as well.
http://www.optionimports.com/hr-porsche-wheel-spacer.html