CourtListener is a free source that does this very well for high-level courts. (i.e., US Supreme Court, Federal Courts, State Courts of Last Resort/State Suprem

CourtListener is a free source that does this very well for high-level courts. (i.e., US Supreme Court, Federal Courts, State Courts of Last Resort/State Supreme Courts). For that, you have to detect references of cases which is a difficult problem itself, and CourtListener's search ranking also takes into account the citation weight of certain cases. This generally works well, but my understanding is that sometimes a not-so-important case can end up having many citations. Or if a case with many citations is overturned completely or partially, these things complicate which cases might be most relevant in search results too. The data source is provided for each case. In some cases, a direct reference/link is provided.