Use Westlaw to find amicus curiae briefs.
Use search tools such as the Think Tank search and Opensecrets to find about organizations filing briefs. Lawyers and groups can often have deliberately shadowed background support that requires some searching to connect.
One way to search the database Westlaw Campus Research for amicus curiae briefs:
Enter the name of the case and search
Look for the words "amicus curiae" in the list of filings. Some are the actual briefs, and some are responses to the briefs.