Most agencies have an adjudicative function where they interpret and enforce regulations. Unlike court decisions, these administrative decisions are not published in one place like a reporter. They might not be published at all.
Administrative decisions are often published on agency websites. A list of federal departments and agencies can be found at usa.gov or in the United States Government Manual.
Subscription Sources (require additional login):
Westlaw Administrative Decisions & Guidance
Lexis Administrative Materials
HeinOnline U.S. Federal Agency Documents, Decisions, and Appeals
The University of Virginia Library has compiled an exhaustive list of places to look for administrative decisions. It's an excellent resource in helping track down administrative rulings.