From December 17 2012, physical products including Battle.net Authenticators will be temporarily unavailable on the Blizzard Online Store in Europe
. This is in preparation for a revamp of the store which will take place during 2013. In the meantime, many items can be found on the US Blizzard Store
Players wishing to acquire a Battle.net Authenticator have several options. The Battle.net Mobile Authenticator is available for many types of smartphone, and you can find out more about it here
. For many people, this is the easiest and cheapest option!
If you prefer to have a physical Battle.net Authenticator, you can get hold of one from several sources including the online retailers and high-street stores listed here