You will have to wait a bit to get tickets online. The website is crap and very unhelpful. I think from last years experience, it took months before tickets were available and even then it was hard to actually buy them. Persevere though as it’s a long way to go to find out they sold out on the day. Id did La Bresse last year, got the tickets online after much hassle. I did see people buying them on the day, but the que was massive and the tickets limited.
You could hike in over the hills for free, but it’s a big effort to do something illegal when you can just buy a ticket. Also you may find you need a wristband to get on the chairlift / access the facilities etc.