Search directly in judicial case databases and Federal Agency decisions databases on Lexis, Westlaw or others. For agency decisions. Hein's Federal Agency Library offers a list of publications that have decisions from many different federal agencies. West's CFR annotated (available through your regular WestLaw Law School subscription, RegulationsPlus) provides selective notes of decisions to CFR sections.
Individual agencies are starting to make their decisions available on their official website as well. University of Virginia Library has compiled a list of agencies websites that contain agency decisions that is worth checking out. When you do a keyword search, using the CFR citation or FR citation as a keyword maybe the most efficient way to find relevant decisions and cases.
Some commercial specialized looseleaf services either in paper or online provide cross- references to CFR or FR citations, such as CCH, RIA Checkpoint etc.