We're not just focusing on cases, but rather tax law cases. Some services make finding them easy, and others take a bit of work. However, below is a good listing of shortcuts to find substantial sources for tax law cases!
Subscription Services
LexisNexis has a simple tax law case collection where specific terms, phrases, judges, parties, or attorneys can be used as search parameters. This is very beneficial as it can help you when you're starting from scratch! Of course, if you know your citation or case name, you can search those here as well. This is the most efficient search engine for tax law cases in my opinion, and helps folks who need information in the most productive and swift way as possible.
Westlaw has a robust collection of tax law cases, though it has a benefit not provided by LexisNexis: the 10 most recent tax law cases entered into the system immediately appear on the page. This is great if you're just keeping up with what has been happening in tax court. There is also a categorized box with tools and resources linking to various beneficial tax code and information collections that really come in handy for extensive or emergency reference. While LexisNexis is a champion of efficiency, Westlaw is a champion of utility in terms of a well-developed user interface designed to make sure you are not stranded when researching whatever you may need.
Bloomberg has court opinion and docket search tools through their tax practice area section of the service. While you can search terms, judges, type of court, and plenty of other parameters, the accessibility is Bloomberg Law's downfall. Thus, though great if you know what you're looking for, this is not as user-friendly as Westlaw or Lexis.
Quimbee provides excellent case brief analysis for a lower-cost subscription, so if you just need to understand a ruling that you've researched then this is a fantastic resource. HOWEVER, this option is only useful if (1) you know exactly which case you need the information on, and (2) the case has been briefed by the company--which is not a guarantee!
The website for the U.S. tax court is helpful for those with a DAWSON subscription, but unless this is obtained there is no way to search. It is not as user friendly as the subscription services, but it is certainly a useful resource if you know the case you need.
Free Sources
The Deloitte-sponsored Tax Notes has a great collection of tax documents, including free tax court, legislative, and treasury documents/decisions for free. While Tax Notes is a subscription service, it is an excellent resource that includes a number of genuinely helpful free materials.
The IRS Website is not friendly to those searching for particular cases, as the only provided court cases are those regarding Indian Tribal Governments. Beyond these few options, look elsewhere if interested in finding tax cases.
While there are other free sources as well for case documents, such as Oyez for Supreme Court cases (which are rare if any regarding tax) and Wikipedia for basic run-downs of incredibly famous cases, these are your best bet for securing accurate and effective information regarding case law.