You can purchase a mountain lion tag for that hunt at any time. It is a general season hunt so tags are available for sale for 2013.
The current season information is copied below. There will also be a season next fall and your tag will be valid through December 2013 (unless you harvest before then).
General Any Weapon
|8/30/12-3/31/13||Either-sex Mountain Lion||Any Weapon||Unit 73³|