I had the same problem on our 2006 365GL back in 2019. Like you, I had little reason to continually open it over the years. Only when I discovered fresh water in the forward bilges with a full tank and heeled over did I find the problem.
I don’t think that Chris Watkins of Universal Yachting, the main Dufour UK agent, holds any spares, although, I have found him very helpful in sourcing direct from Dufour in France in the past.
However, the good news is that I sourced direct from the French manufacturer, L’Oceane des Plastics. You’ll find their name inside the inspection hatch. I googled the company name on the internet. I then emailed them at email@example.com
Anne Gautier, firstname.lastname@example.org responded in English with a quote for a Bouchon Rouge + joint, part number 9000002, priced then at Euro6.50.
The bad news is that there was a post and packaging cost of Euro 15.29 and a financial fees charge of Euro 25.00 for a small order.
I ordered 2, so that I had a spare for the aft tank, should that ever need replacing. I had to pay a proforma invoice sent by email. Goods were then promptly dispatched, fitted and all good.
Hope that helps.