You can find the FEC’s brief here.
Your can find the McCain et al. intervenors’ brief here.
You can find the Briffault and Hasen amicus brief supporting the FEC and McCain here. I’ll update this post with more amicus briefs if links (or briefs) are sent to me.
UPDATE: An amicus brief for the League of Women Voters et al. (and authored by Dan Ortiz) is here.
Here is a brief from four former ACLU directors, and written by the Brennan Center.
Here is the amicus brief for the Committee for Economic Development, Norm Ornstein, Tom Mann, and Tony Corrado