Tax cases are heard in domestic courts and in some regional courts, such as the European Court of Justice, on appeal from domestic courts. So, in general, you need to use databases of domestic cases to find tax cases in the jurisdiction in which you are interested.
The IBFD tax research platform includes over 1400 cases from a multitude of countries. You can search via country/jurisdiction, related country/jurisdiction/region, international organisation and a range of topics and sub-topics, such as Corporate taxation, Treaties, Transfer prices, etc.
To find cases in domestic courts on international taxation issues using IBFD, filter by Individual Taxation > Taxing Jurisdiction > Source > Domestic
Many jurisdictions also have specialist tax cases reports or databases on commercial platforms (available to UniMelb staff & students) containing selected tax cases. For example:
Canada Tax Cases and Decisions on Westlaw Next Canada contains Canadian federal and provincial reported and unreported court and tribunal decisions (including those of the Tax Court of Canada) relating to taxation, including federal income tax, federal and provincial goods and services taxes, and other provincial and territorial taxes. The earliest reported decision is from 1827.
Dominion Tax Cases - law report series covering federal and provincial court decisions on federal tax matters - cases from 1920 to 1956.
Canada Taxation Digest on Lexis + Australia - contains tax case summaries from all Canadian common law jurisdictions, organised under more than 1500 taxation headings and sub-headings. Within each heading, the summaries are organised by court level, then by jurisdiction, and then by date. This source can be browsed or searched.
Canadian Tax Reporter Commentary on CCH Online Canada includes Recent Case Digests (2003 to current) and Commentary - which is arranged by topic and refers extensively to cases on point.
New Zealand Tax Cases (vol 1, 1973 to current) on CCH Online Australia includes selected significant court decisions and Taxation Review Authority decisions. The full text of every decision is included, preceded by a headnote summarising the facts and the court's decision. Also included is an Appeals Pending table to keep track of important decisions.
Tax Cases - Westlaw UK contains case analysis from Tax and Chancery, the Supreme Court, the Tax Chamber and the Civil Division. Holds cases from 1860- 2014.