Aug 19, 2017 - 9:56 AM
Welcome to MySwissAlps Athmika!
All purchase options, on- and offline, are listed here: www.myswissalps.com/sw isshalffarecard/price.
So you can either buy it at a staffed Swiss train station during opening hours or buy it online. Buying online is more convenient. You will receive a PDF file to print. The printout is a valid Swiss Half Fare Card, and the file is a backup to print again in case you lose or damage it (you won't have that if buying at a station). Also in your case you would have to get off the train at the border to buy the Swiss Half Fare Card and the ticket to Wengen, and wait for half an hour to an hour for the next train.
I would simply buy it online, e.g. through www.swissrailways.com/ en. After filling out your address, they will show the delivery options. Pick "Online Ticket" from the bottom of the list. They will e-mail the card within 1-3 business days (usually less than one day). The discounted ticket can be purchased online as well.