As has been reported accurately, tickets cannot be purchased until 60 days from the desired date at the earliest. And they indeed sell out within a few days.
What is not known is that there appears to a prioritization for who gets to buy the tickets first, once the 60 day mark is reached. I waited until what would have been 12 midnight in Paris of the day it should have been possible to purchase tickets. Even though I re-tried for the next 30 minutes on the official website, they remained unavailable. That was as an individual. Just to see, I clicked on the professional category and it was entirely different. Tickets for my selected date were available, though even within a few minutes of logging in, they were gone for most of the time slots otherwise available. I was able to select the date and time I wanted, but of course I wasn't registered as a professional so I wasn't able to purchase the selection (I assume that's the reason). Having a specific date in mind, and relatively so for the time of that day; we ended up purchasing a tour, costing about 1/3 more than the direct purchase would have. The extra cost isn't so much the issue, it's people need to know how ticket purchases appear to be prioritized and not to expect to get the tickets they want even though their effort is made in a timely manner.