You have to keep your eyes peeled constantly on the secondary market, but they are out there. Shooter pricing are generally considered $250-550 depending on desire and overall condition. I picked up a 67 PPK in .380 last year and paid 650 shipped. More than what I wanted to pay by $50 but I really wanted a West German PPK that I could mess around with to my hearts content and used ones don't come on the market as often, and when they do, they are quickly bid up to unreasonable prices for used guns.
I couldn't be happier with the purchase though. Despite appearing not having been cleaned since 1967, containing more lint than you can possibly imagine, and having about 50% of the slide bluing worn off from years of carry, after giving it a deep cleaning, I was very pleased to see that it had been shot very little. The bore is mirror bright and the barrel shows barely any finish wear, with all of the internals being in great shape.
So they do come up from time to time. Stay vigilant.