Hello Conguttack21
Welcome to the SBB Community.
Best to purchase it from the train operator, especially if you hold the Swiss Travel Pass in addition:
https://www.jungfrau.ch/en-gb/jungfraujoch-top-of-europe/buy-jungfraujoch-ticket/
We do not recommend purchasing mountain excursion tickets days in advance. Only purchase it the day before or on the day of travel. It can also be purchased at the BLS train station in Interlaken, with the ticket agent.
Kind regards,
Roland