http://www.porscheforsale.org/
autotrader
autotraderclassics
aol
yahoo
and cars.com Most of these sights will let you save searches so with one click you can see what you are looking for, saves a lot of time.
I found my carrera on ebay- no PPI, it was my first car and I only looked for about 3 months. I bought it knowing it needed a top end and have now found a few other issues that I will someday have to deal with. I could have found a better car if I was more patient and did a PPI but I have learned a lot and it has a very nice-interior. I am lucky the car did not have more issues.
I looked for my turbo for 6-8 months. Found it on auto trader. It was the 3rd turbo I PPI'ed. PPI's are a must, don't even think about buying a car without one. Also do some research on who is going to do the PPI.
The other thing I found; there were a few cars I looked at and initially thought they were what I wanted. But the more cars I saw I realized you really have to be patient and not make a quick decision. During your seach you will change your mind as to the car you want. There was one car I really liked except for the exterior color. Looking back I am really glad I did not buy that car. You may miss one nice car because you did not pull the trigger but another one will come along, and this is better than getting the wrong car. I have 2 P cars and wife would have no problem shooting me if I bought another, but that does not mean I don't look. I have short cuts to all kinds of sights and it is fun to just see what is out there.