As noted in this guide, not all docket materials are available online or in digital formats. In addition to the free/online, low-cost/fee-based, and sponsored/subscription resources covered in the rest of this guide, see the Law Library and nearby resources for briefs in print and other "in-person" or onsite formats; some holding institutions offer scan-and-deliver services (usually fee-based).
Supreme Court briefs for recent and high-profile cases can be easy to find, including often via free online sources (see SCOTUS briefs under "High-profile & historic briefs from federal courts" under Online Dockets & Briefs). However, older briefs and those from lower-profile cases or cases where certification is denied—and in particular, in forma pauperis cases where certification is denied (these are transferred to the National Archives at the end of each term)—are not easily available.
For federal courts of appeal materials not available online, several other libraries may have other formats.
A number of law libraries offer access to California Supreme Court and appellate briefs that are not available online—holdings vary, so contact the libraries for details about years covered and format.