My thought on the matter is that most people will not know that they have a ZHP, or a ZSP for that matter.
You need to harvest the VIN numbers of cars that appear to have ZHP styling -- lights, tire packages, aero stuff, those kinds of things -- and plug the VIN into a decoder to see what the original option packages were applied by the factory. Someone can easily (20 years later) be selling a ZHP and not know it. Clearly there will be enthusiasts who know all about the ZHP/ZSP, and they will try to up charge for the options, but unless the car is a pristine specimen, I don't think that the option package that it was originally equipped with is worth very much today. Maybe you are finding cars that are priced high because the miles are very low, and this gives value to a greater degree than the options package it might carry.
I would suggest you use sites such as cars.com or autotrader.com and search for the years when the ZHP was offered, and see if there are people selling them and not knowing. You should be able to identify a ZHP/ZSP by the picture, then confirm by the VIN. The ZHP and ZSP (the ZSP is similar, but without the factory engine tune) that should have unique options that can't be easily swapped into the car. For example, anybody can find a set of 18-inch wheels and put them on a car, but installing the Alcantra interior is not really a thing that most people will do. If you see a car with factory 18s and Alcantra, the odds favor it also being a ZHP, but the VIN decoder will nail it. The ZHP and ZSP all exist among the sedan, coupe, and convertible, and I'm not certain but I think the M54 engine (325/330 -- and maybe only the 330) is required. Since you are looking for the 330, you might not even be interested in the 328 ZHP, if there is such a thing. My point being that you might have a ZHP across town that somebody is selling, but they don't know what they have so they list it as an '05 330 with 18s and a 6-spd transmission, and you run the VIN through the decoder (mdecoder.com is a good one) and find that it's your dream car.