4-piece set tickets Airport Bus Sapporo

I learnt from the web page that I can purchase a 4-piece set tickets when travelling 4 people together in Airport Limousine Bus from New Chitose Airport to downtown Sapporo for 3710 yen. Normal is 1030 yen per person.

May I know where can I purchase this ticket? Upon arrival at the airport at the bus station or can I purchase it directly when getting into the bus? Thank you and have a nice day.

http://www.chuo-bus.co.jp/highway/airport/ticket.php

4枚つづり回数券: 4-piece set tickets
大人片道1,030円区間 3,710円（1枚あたり928円）:Valid for 1030 yen route 3,710 yen (928 yen for one ticket)

小人は半額です。: Half price for Children
北都交通運行便でもご利用いただけます。:Valid also on Hokuto Kotsu bus
発売箇所: Where to purchase
中央バス札幌市内各ターミナル窓口、新千歳空港カウンター及び空港連絡バス車内
Chuo bus Sapporo terminals, Chuo bus Shin-Chitose ticket booth and on the Airport bus

So you can purchase it on the bus, probably when you get on the bus, from the driver.

Thank you frog1954 for the great information. Have a nice day.