You might think that these are all online, but they are not! Here are the briefs research guides I use:
1) Tried and true, if you are near a library with this print set: Gerhard Casper and Kathleen M. Sullivan, eds., “Landmark Briefs and Arguments of the Supreme Court of the United States: Constitutional Law”
3) Official U.S. Supreme Court website has several links to sources of briefs.
4) Guides: LLRX Supreme Court Web Guide (including a section on briefs)
5) Run a Clusty, Dogpile, and/or Google search using the name(s) of the parties and the words supreme court briefs.
There are other guides, but these are the ones that have worked well for me in the past.