Is there source of U.S. Supreme Court record items (merit and amicus briefs, appendices, etc.) from 1942?
I'm interested in a general answer, but in particular, I'm interested in the record from Wickard v. Filburn, 317 U.S. 111 (1942).
To my knowledge, there is no complete online archive of historic US Supreme Court documents.
There are links to recent indices of online material from http://www.supremecourt.gov/default.aspx, and the court actually redirects seekers of online merits briefs to the American Bar Association's "Preview" publication A direct index of cases is available for 2012-13 and 2013-14; for earlier material, PDFs of Preview can be downloaded, but only back to the 2007-08 session.
The FindLaw site has online briefs dating to 1999.
I don't have access to WestLaw, but the SCOTUS site suggests that it may have historic briefs. If so, I would not assume that they have them as far back as 1942.
Otherwise, I believe you'd have to visit a law library or use a commercial document retrieval service, which would send a person to the court to make copies of the relevant documents for you. This page lists libraries which are repositories for printed briefs as well as listing services which can retrieve documents for you (for a fee).