Do police depts. buy directly from Ford ? or a local dealer ? If a dealer, doesn't the dealer own the car ?
Interceptors are ordered from Ford through the dealer and are delivered through that dealer. As mentioned upthread, Ford will not knowingly sell the police interceptor to a member of the public, so Ford is involved at that level early on.
On the other hand, if you have a friend in a dealership he can search inventory computers for PI 'misorders' (which are new but, not matching the order properly, can be sold to anyone who wants it).
A new police interceptor is a very good thing indeed (though some amenities like cruise control and stereo will be missing and must be added on).