The iTunes Store is a great place to find episodes of TV shows that you might have missed, or even catch up on past seasons, but it's only one of several options available.
Netflix offers a free iPad app to watch movies and TV shows for those who have signed up for their $8/month streaming service (or higher). An iPhone/iPod touch app is also available. Netflix still has more shows available via DVD than instant streaming, but there's still plenty there.
Hulu has a free Hulu Plus app
