Hello Whauckiettuett83
Firstly, welcome to the SBB Community.
Albeit SBB sells it, the Tell Pass is a private offer (not from SBB).
It is not available as flex pass. You can choose from two up to ten consecutive days.
You can purchase it online or at the SBB travel center, for example at Lucerne station, or at the Lucerne tourist office.
I, personally, do not see great value in the Tell Pass. It really depends on what you plan as activities.
Alternatively, you could use the Swiss Travel Pass or the Saver Day Pass, or the Swiss Half Fare Card and individual tickets - all depending on your planned itinerary in Switzerland.
Our advice is to complete the entire itinerary planning and, as the last step, to decide on travel passes and tickets.
Kind regards,
Roland