^Can't find it either on parcasterix.fr for the moment.
Or they don't sell it any longer or it's too early to buy at the moment. You can try to call the park or wait a bit longer and check back regularly.
I remember when I bought them, you had to choose and fix the date you will be visiting. You can buy the tickets upto 3 or 5 days (I don't remember exactly) before your visit. If you have a timeframe of a few days for your visit, you can wait till the last minute and decide based on the weather forecast.