The Constitution
While its unlikely you will ever cite to it, U.S. Const. art. I, § 8, cl. 1. gives Congress the power to collect taxes. For an interesting article on the limits the Constitution places on Congress' power to write tax laws, and some of the legal challenges that have been made, you can check out: Stephen W. Mazza & Tracy A. Kaye, Restricting the Legislative Power to Tax in the United States, 54 Am. J. Comp. L. 641, Westlaw (2006).
Federal Statutes
Title 26 of the U.S. Code is reserved for federal laws regarding taxes. You'll often hear this part of the Code referred to as the "Internal Revenue Code" or the "I.R.C." If you're hunting for a code section on a specific issue, refer to one of the practice centers or other secondary sources which are organized by topic. Bluebook Rule 12.9.1 allows citations to the Internal Revenue Code to use "I.R.C." rather than "26 U.S.C.," so be aware that these citations are to the same part of the code. The Bluebook: A Uniform System of Citation R. 12.9.1, at 130, (Columbia L. Rev. Ass'n et al. eds. 21st ed. 2020).
One additional thing you should note is how major tax reform can effect the tax code . For instance, the Tax Cuts and Jobs Act of 2017 was a major update to the I.R.C., and any case law or other source which deals with issues before tax year 2018 may have substantially different versions of the code guiding those decisions.
Federal Regulations
The United States Department of the Treasury, which includes the Internal Revenue Service, interprets the Internal Revenue Code by issuing Treasury Regulations. Final versions of these regulations are published in the Federal Register and later codified in Title 26 of the Code Federal Regulations (C.F.R.). Like the Internal Revenue Code, sections in Title 26 of the C.F.R. can also be referred to as the Treasury Regulations and will be cited to as "Treas. Reg". The Bluebook: A Uniform System of Citation tbl.T.1.2, at 235-26 (Columbia L. Rev. Ass'n et al. eds. 21st ed. 2020). Unlike some areas of the C.F.R., the regulations on tax generally correspond with the section numbers listed in the U.S. Code. For instance, to find regulations which interpret I.R.C. § 162, which covers trade and business expenses, you would look to Treas. Reg. § 1.162-1, 2, 3 . . . etc. Note that the first number in the C.F.R. section corresponds to its "Part" in the C.F.R.. Regulations pertaining to tax procedure are found in Part 301. Thus, the regulation which interprets the general statute of limitations on actions for assessments and collections of tax returns, I.R.C. § 6651(a), is found in Treas. Reg. § 301.6651(a)-1.
Case Law
Case law on federal income tax issues usually comes from multiple sources: the federal court system, including the Supreme Court, Circuit, and District Courts and the United States Tax Court. These courts interpret the laws written by Congress and the regulations written by the Treasury Department.
Official Sources
The official United States Code is available at https://www.govinfo.gov/app/collection/uscode.
Annotated Statutes
Westlaw, LexisNexis, and Bloomberg Law each have annotated versions of the Internal Revenue Code. You can use these sources to find major decisions, cases which cite to the code, amendments, prior versions, and even legislative history.
Free Sources
Cornell's Legal Information Institute maintains a free online version of the U.S. Code. Here you can look at the text of the statute along with editorial notes, such as references and amendments.
Tax Notes also offers a free online version of the Internal Revenue Code in their free resources suite.
Official Regulations
The official Code of Federal Regulations can be found www.govinfo.gov/app/collection/cfr.
The official version of the Federal Register can be found online at https://www.govinfo.gov/app/collection/fr.
Commercially Available Regulations
Westlaw, Lexis, and Bloomberg also provide commercial versions of the C.F.R., but they are not annotated like their statutory contemporaries. You can also use Westlaw and Lexis to search for documents published in the Federal Register, and if you are using Lexis+, you can browse through the Federal Register by year in its dropdown menus.
Free Sources
The e-CFR, while not official, is free resource you can use to locate regulations. It’s much easier to navigate around in than the official online version, and is “complete” in the sense that you do not have to hunt around for regulations which might be “missing” in a year that you are browsing.
Cornell’s Legal Information Institute also maintains a free online version of the C.F.R..
You can also find a free online version of the Treasury Regulations in Tax Notes' free research suite.
When hunting for relevant case law, you want to keep in mind that the sources come from the federal courts. If you’re using a commercial database like Westlaw, Lexis, or Bloomberg, you can easily limit the jurisdictions to “All Federal,” and uninclude state jurisdictions.
To generate relevant key words, look at the language that is used in the practice centers for your taxpayer’s issues and the relevant statute numbers. By combining these phrases, keywords, and statute numbers, you can put together natural or Boolean searches to find cases which are relevant to your taxpayer’s issue.
If you are still having trouble finding good case law, look to cases which are cited in the practice centers, secondary sources, or annotated statutes.
If you do not have access to any of the subscription services, Tax Notes maintains a database of tax cases which you can access for free.
If you find your tax issue is in an area of law that is not well defined or ambiguous, you can look for answers in the legislative history. The background and events leading up to the passing of a statute may provide insight into what Congress’s intent was. Much of this information is available online. For a helpful guide on using legislative history in federal tax matters, you can use The Blue Book, Portfolio 100-3d, Bloomberg Law.