Hi, Graham Cooke (Graham.Cooke)")!
Thanks for reaching Microsoft Community and the interest to find a charger you can use in the car. We have a Griffin Surface Link Vehicle Charger but currently, it is out-of-stock. Please contact Microsoft Support and connect with Microsoft Store to check availability.
If there is none, you may try other online shops. Ensure that they have the same specs with the in-box charger of the Surface Pro 6, which is 15 volts @ 2.58 amps = 39 watts with total wattage of 44 watts.
We hope this helps! We highly encourage you to inform our Community by clicking on Yes or marking my post as an answer if this answers your inquiry. This will help other members who may have the same issue as you. Otherwise, don't hesitate to update us so we can further assist you.
Kind regards,
Janette_Z