Statutes are created by the legislative branch of government and organized be subject in the Unites States Code. Statutes are also accessible in some research databases. Here are links to a few access points.
The Internal Revenue Code (United States Code, Title 26) contains the statutes dealing with income, estate, gift, excise, and employment taxes. The first IRC was passed in 1939, with subsequent revisions in 1954 and 1986. Tax law can also be found in the Bankruptcy (Title 11) and Criminal Law (Title 18) sections of the USC.