From the PACER website:
Public Access to Court Electronic Records (PACER) is an electronic public access service that allows users to obtain case and docket information from Federal Appellate, District and Bankruptcy courts, and from the PACER Case Locator via the Internet.
Each court maintains its own databases with case information. Because PACER database systems are maintained within each court, each jurisdiction will have a different URL. Each court service is comparable to the others; however, the format and content of the information may differ slightly.
PACER is a service of the United States Judiciary. The PACER Service Center is operated by the Administrative Office of the United States Courts.
Note: Because there's often a charge associated with using PACER, we do not distribute the password. If you are unable to find what you need using Bloomberg Law (first choice), Lexis Advance CourtLink, or Westlaw Dockets, please consult a reference librarian, who can check PACER for you.
Justia doesn't provide docket sheets, but it can be a good, free starting point for finding dockets for the federal district and appeals court. The search engine is more user friendly than PACER's, so it can be useful to come here and find a docket number and other basic information on your case. Coverage begins in 2004.