The International 2 (2012) event tickets will be available at Dota 2 Store for 49.99$. It will allow you to enter the event venue throughout the 3 days of the tournament from August 31 to September 2. The tickets will go on sale on 12th June. Once you purchase it, it will be placed inside your Dota 2 loadout inventory which will provide you event access pass and other details via email.
If you are unable to attend this big event, You can always watch the Dota 2 International 2012 through Live Stream/Dota TV.