Heres's a list that will keep you very busy. Good thing it's winter!
Porsche For Sale Links
No affiliation, but a decent listing I came across a while back. Some listings may need updating.
You also should check PCA.ORG. If you are not a PCA member you can still access the 'Regional' chapters, some have their own classifieds and most of those listings aren't listed anywhere else for some reason. Some of the best kept cars are here, rennlist & pca. Craigslist can be a bit of a crap shoot from my experience.
Go for the best car your money can by, but know what you like. The point about the 914 is excellent advice. No matter what you buy it will cost you again in ownership, so just save enough to have on hand to deal with it.
Just my 2 cents.
Rob