Click on the ‘My Nation’ link (in the left hand column), then login to the site using your username and password (the same as you use to login to the game). There is a tab called ‘SAGA Store’ that allows you to buy (or redeem already purchased) booster packs.
You can also access this page by clicking on the various ‘buy boosters’ links from within SAGA itself (in the Banner menus, from the Market and from the Manage Booster Packs link at initial login). You may need to logout and log back in if your new cards don't post immediately.
