I bought my 2012 4S from a non-porsche dealer. They gave me an exclusionary warranty 4yr/48k w/ $100 deductible from Ally (Major Guard). However, I can't speak to the quality of the warranty service as I've never used it; part of me hopes that I never have to use it!! It did give me piece of mind though to make the purchase (dealer did their job).

Apparently, I'm free to go to any Porsche dealer for work and they'll pay it. The Porsche dealer near by says they've dealt with 3rd party warranties and, generally, they have no issues.

So far I LOVE my 4S and I have no regrets on the purchase. I bought it with 34k miles for $48k; it's in excellent condition and came very well equipped.

Good luck with your search. If you haven't already, check out cargurus.