I'd suggest looking at PCA (
https://mart.pca.org) for a good site with reliable postings. These folks are real Porsche fans, and I expect most of them take care of their cars and keep good records.
At the other end of the spectrum, there's craigslist.
In any case, ask for whatever records are available, a CarFax, and most importantly, a PPI. Even an honest seller may not be aware of everything about their own car.
Good luck!