Liftpasses can be booked online or purchased in resort, however it will be cheaper to purchase with your accommodation, if booking through as they will have various incentives to do so including booking it early. 

2017/18 Example 6 day liftpass price (rrp)
Adults (18 or over) = 373 chf. 
Teenagers (13-17) = 248.60 chf. 
Children (6-12) = 124.30 chf. 
Kids up to five get free liftpasses
More prices here