Nexis Uni is a source of state and federal cases. Searching available by topic, involved parties, or by citation.
WestLawNext is a source of federal and state cases. The KeyCite® tool enables users to research whether a case is still good law.
For specifically tax cases:
Try:
CCH IntelliConnect Source of Federal & State Tax Cases especially by topic.
Under All Content, Federal Tax => Federal Tax Primary Sources => Cases
Under All Content, State Tax => State Tax Reporters => Massachusetts State Tax Reporter.
Bloomberg Tax is also a source of federal tax cases.
On main page, scroll down to =>Cases & Court Rules to view Federal Cases.
Links, and instructions for accessing them, are available for both CCH IntelliConnect & Bloomberg Tax on the Databases A-Z listing. For assistance with the above databases connect with a Reference Librarian.