The only place to buy them is from the Ventra Vending Machines at the entrance to the train station. They are where the man in this photo is standing:
http://www.transitchicago.com/assets...recontrols.jpg
Just follow the overhead signs that say "Trains to City/CTA" and they will guide you to the station.
You purchase the Ventra Card and the pass in a single transaction. Press the button on the home screen of the machine in the lower left corner that says "more options" and then press "New Ventra Card." It will ask you what you want to load the card with, select "Pass."
A 7-day Pass will expire 7 days after the first time you use it. The Ventra Card will expire approximately 5 years after purchase, so you can keep it for future trips to Chicago. If there is a balance on the Ventra Card, there will be a $5 a month inactivity fee after 18 months. (The inactivity fee does not reduce the balance below zero. You will never owe them money.)
If you want a 1-day pass, you have two choices: You can buy a 1-day Ventra *ticket* from the home screen on the Ventra machine. A 1-day ticket does not require a Ventra Card. Or you can buy a Ventra Card and then add a 1-day pass to the card. All passes longer than one day require you to have a Ventra Card.