Hmm, looks like HP doesn't have one. Their auto adaptors only go up to 90W and the ZV6000 needs the 120W version.
Also, you're not looking for an inverter; they take DC and convert it to AC.
You could get one of the Carnetix DC-DC regulators at the MP3Car Store. The CNX-P1900 is about what you'd pay for a simple auto adaptor but it has many more features and is better suited to the car environment. It doesn't do exactly 18.5V but that shouldn't be a problem.