Administrative law refers to law created by the executive branch of government. In the U.S., federal agencies receive authorization from Congress in the agency's enabling act to create regulations and to issue agency decisions. Agencies can have both quasi-legislative powers (the ability to promulgate regulations which have the force of law) and quasi-judicial powers (the ability to hold hearings and issue rulings on matters within the purview of the agency).
Federal administrative law - The major sources of federal administrative law are described in this guide and research sources are listed. Both rulemaking and adjudicatory activity of federal agencies is described.
State administratve law - Major legal research vendors: BloombergLaw, CCH Intelliconnect, LexisNexis and Westlaw cover state administrative law. Coverage for smaller state agencies may be limited, particularly for agency decisions. A review of the particular state agency's website may offer more detail.
The Administrative Procedure Act (APA) governs how federal agencies propose and promulgate regulations. It also establishes how federal courts can review agency decisions. Passed in 1946, the APA can be located at 5 U.S.C. 500 et. seq or Pub. L. 79-404.
In general, each state will have some version of an Administrative Procedure Act. To locate these acts, one could use the Popular Name Table or the index of a state statutory code. One could also perform a search within various electronic resources such as Westlaw, Lexis Advance, Bloomberg Law, or a state legislative website. For example, Massachusetts' Administrative Procedure Act can be located on the state legislature's general laws section of the website.