These are links officially linked to Banksy,
banksy.co.uk – Banksy’s official website
Pictures On Walls is the last place to sell official Banksy Prints to the public. At the time of writing there is nothing available but you can sign up to their mailing list.
Banksy’s Pest Control Office is the only place that can officially authenticate his original artworks.
Banksy also states on his website that he is not on either Twitter or Facebook but this banksyfilm youtube account is worth a watch. He’s also previously used Instagram when in New York.