Bill Sponsor information can be found in the Bill Profile Reports found in ProQuest Congressional.
Click on "Bills & Laws" under Document Types on the left. Then you can enter the bill name and year. You can also restrict the search by status if, for example, you only want to search bills that became law. (Restricting the search to bills that became laws will not be helpful in some cases, such as when the bill number that passes is a large omnibus bill)
Then that will give you the Bill Profile Report
In the Bill Profile you'll see the tab in the center called "Sponsors"