From your album page, you have the option to view 'Archived Sets' by clicking on the 'Go to archived sets' tab at the top left of the page. The current archived sets are the first 50 sets published. You can still purchase these cards through their own store, in trade, free sends and by trading with friends. Sets will be archived on a regular basis, to keep 'current' sets at a manageable level and not to overwhelm newer players.
You can also buy cards for these sets through the store; there is currently 1 page of antique cards in the shop.