U.S. House of Representatives

U.S. Senate

House Ways and Means Committee

Senate Finance Committee

Thomas is provided by The Library of Congress as a way to find legislative information

The U.S. Code as provided by the Office of the Law Revision Counsel, official publisher of the Code.
See also the Table of Acts Cited by Popular Name. 

U.S. Department of Treasury

Internal Revenue Service

IRS Tax Statistics

Congressional Budget Office

Government Accountability Office

Taxpayer Advocate Service

Office of Tax Policy