You can purchase one anytime before the 12 month warranty runs out. You can get one for 24, 36 or 48 months. You can contact any dealer or check http://www.rongreenesportscenter.com/ they have them much cheaper than the dealer.
I bought my bike new in Feb 04 and didn't buy an extended warranty at the time and after being a member of this group and the Yahoo group I decided I could handle about 99% of any problem I may have with the bike and would save the money for parts etc. If you do buy an extended warranty you will either need to make sure the scheduled maintenance is done at the dealer or keep recipits and records of when you do it.