Browse the current budget and previous budgets: http://www.gpoaccess.gov/usbudget/
Visit the Web site of the Office of Management and Budget, the White House office responsible for devising and submitting the president's annual budget proposal to Congress: http://www.whitehouse.gov/omb/
See also the Center for Strategic and Budgetary Assessments, http://www.csbaonline.org/, an independent, non-partisan policy research institute, that posts interesting reports and analyses of budgetary matters.
You will find the most recent paper copy of the Budget of the United States Government in the NWC Reference Collection, Ref HJ 2051 A5, as well as volumes on Historical Tables and Analytical Perspectives close by on the shelf.
