If you are looking for a particular tax source:
These are arranged alphabetically by abbreviations. The abbreviations are followed by the name, law library call number, and free online source. Most tax sources can be found on Bloomberg Law, CCH Intelliconnect, LexisNexis, RIA/Checkpoint, and WestlawNext. For a clear quick refresher on the sources see the Internal Revenue Manual section entitled “Researching Tax Law” 188.8.131.52.
*Note, many tax sources are not “published” by the government entity that creates them. When this is the case, start with either CCH Intelliconnect, Bloomberg Law, or RIA/Checkpoint as these commercial publishers have long specialized in gathering tax law information.