Eurostar/Paris Metro

You can buy them from the buffet counter, or from the information desk in the post-security departure zone at St Pancras.

Thanks, Philip. That does sound more convenient/less crowded than the Gare du Nord Metro station.

Thanks so much Phillip.

The only tickets sold, either on board the train or in the departure area at St Pancras (far right hand wall, midway along the wall towards the back near the Cafe Nero coffee) are carnets and Paris Visite.

If you want Mobilis or anything else you need to get it in Paris, downstairs.

Sorry, I didn't notice you were asking about one-day passes.

No, unfortunately you can't get those on Eurostar. If you are only doing a day trip, it might be worth waiting and getting a one-day ticket. If you are staying in Paris for more than one day, carnets (packets of ten single-journey tickets sold at a discount rate) are better value than a one-day pass unless you are making more than four complete journeys in a day (remembering that you can change between bus routes or between Metro lines free on a single ticket).