Q: I am in the process of purchasing a 356 and was wondering where I could find out the market values of the vehicles.
A: The two best sources in print are the PCA Magazine Panorama and the Porsche Market Letter (PML).
To get Panorama you must be a Porsche Club of America (PCA) member. You may have a friend in the PCA who can loan you a back-issue which gives the "Market Watch" for the models in which you're interested. They rotate different models through each issue, so you have to find the right one.
The PML is subscription-based. See http://www.pmletter.com/ I believe their September issue specializes in the 356. Like the Market Watch in Panorama, they list trends in prices as well as samples. You have to subscribe for a year, but as your search for a car might take that long, it's also a good source of classifieds, as is Panorama.
There's also the National Automobile Dealers' Association (NADA) book of used car prices. See http://www.nada.com Note that I found in my search last year that their prices tend to be lower than the actual market by 10-20%; much more on the rarer cars, like the Speedster.
Finally, I have to put in a plug for the 356 Registry. You're not going to find a better source of information on the Porsche 356. The cost of membership gives you a superb magazine of which the ads are worth the full price alone. I've been a member two years and I'd wished I'd joined 10 years before I bought my 356. I was a member for a year before I bought my car and in that year I'd learned more about what makes a good 356 for purchase in the previous 20 years I'd been wanting one from a distance.
Good luck with your search for your own Porsche 356!