how do i buy iWork from apple store

Assuming you are running OS X 10.6.6 or later (I'm guessing you are as you're in the Lion 10.7 forum 🙂) you should have the 'App Store' in the Apple menu (also in your Applications folder). Launch that and it should become apparent - I think it's prominently displayed on the featured page. You can buy the three apps separately if you wish.