Buying a Servive on the Internet

What kind of "services"?

I wish to find postal addresses of various Internet sites when they are not listed on the site.

In some cases I have been able to find them through what I think is the "who is" function of the Registrar. But I don't know how to figure out who the registrar is for a given site or whether the address provided is phony.

You don't need to know who the registrar is to get the whois information.

As to determining whether or not the address is real it sounds like a job for a PI and I don't know of any reliable resource to have PI's bid on work against each other.

Why not do it the usual way and get quotes.