Hello Nestler
Either is possible. You can purchase if from sbb.ch/en, indicating Zürich HB to Interlaken Ost via Luzern. Or, purchase it in the SBB travel office upon arrival. SBB as well as sbb.ch/en can sell you any public transit tickets for Switzerland, no matter the operator. That makes our public transport system so successful.
Whether a point-to-point ticket is the best offer, depends on your travel plans/needs in Switzerland. It is advisable to calculate all travel costs upfront, to see if a pass would be more beneficial. Buying point-to-point tickets looks easy yet is not the most economic in certain cases. Also, Supersaver tickets are not a good idea when arriving by air; they do have a fixed departure time and when you miss it due to a late flight arrival, they are lost.
Kind regards,
Roland