Tax law, like law generally, can come from one of four place: Constitutions, Statutes, Courts, and Administrative agencies.
For example, in Illinois tax law can be found in Article IX of the Constitution; Title 35 of the Illinois Code; the Independent Tax Tribunal, the Illinois Department of Revenue and the state's appellate courts
Additionally there exists a multitude of secondary sources including 50 state surveys, treatises, looseleafs, law review articles, news and policy analysis to assist a researcher in understanding and analyzing the law. In this guide you will find links to and descriptions of these resources.
State and Local Taxation and Finance in a Nutshell by 