We recommend Spot Hero (available via the web or an app) for finding parking.
One of the closest parking locations, managed by iParkIt, is:
Willis Tower Self-Park Garage
211 W. Adams St.
This 24-hour self-parking facility is directly across the street from the Willis (né Sears) Tower.
Start at www.iparkit.com.
Click "Event Parking" then "Chicago", then "JW Marriott".
That should show a $20 rate for each 24 hours, with full in-and-out priveleges. Just searching for the same parking spot will be $55 a day, so make sure to use the "Event Parking" method.
Upon purchase, you will receive a bar code pass (prinatble or mobile-friendly) to be used for entry and exit. Do not take a ticket when entering the garage, simply scan the barcode.