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HeinOnline has several libraries which can be of enormous help when compiling a legislative history. There is a library for the Code of Federal Regulations, Federal Register Library (which contains the Daily and Weekly Compilation of Presidential Documents), Session Laws Library (covering all 50 states), the United States Code, U.S. Congressional Documents (includes Congressional Globe from 1833-1873, Congressional Record from 1873-2009, and Congressional Record Daily from 1980-2012), and U.S. Federal Legislative History Library (contains selected compiled legislative histories).
HeinOnline contains an online version of the Statutes at Large and can be a useful resource for locating the text of the Public Law.
HeinOnline also contains the Daily Congressional Record; that can be used to locate Congressional Debates.
In addition, HeinOnline can be used to locate Presidential Documents (presidential statements in regard to signing legislation, sending proposed legislation to Congress, or vetoing legislation). This is found via the Weekly Compilation of Presidential Documents (available for 1965-2009 through the Federal Register Library).