The ticket e-shop allows the sector selection when you open the item "EVENTIM Seating Plan." In this case, the link opens an active plan of the Vysočina Arena.
Clicking in the active Arena plan on respective sector you choose your desired location. If any of the sectors is already sold out, you'll recognize easy. It will be colored gray and inactive. Then you must choose one of the active items, which are highlighted in yellow (or purple for VIP). Demonstration of how to get to the "EVENTIM Seating Plan" and how to choose the sector brings you also the following graphics (click to enlarge).
And one advice in the end. Many of us organizers were surprised by how unusual view of the stadium offers the sector E! It is definitely not the last sector in line, it rather offers an unprecedented overview of the events in the Arena without turning heads. You can watch a very long part of the track before the arrival to the stadium as well as pretty much everything else in the stadium. Also, you can only enjoy this view on exceptionally large occasions like the Biathlon World Cup. In addition, during this year's World Cup, in front of the sector E will be again installed a big screen for the best view. Assess by yourself!