From what a google book search finds me, I am seeing references to it in G. Lenotre's "The Last Days of Marie Antoinette" from 1907, Hilaire Beloc's "Marie Antoinette" from 1909, John Richard Hall's "The Bourbon Restoration" from 1909, the aforementioned Loomis book. Deborah Cadbury's "The Lost King of France" (2003) says not only was the letter found under his mattress, but as were assorted locks of hair and other mementos.

Stefan Zweig's "Marie Antoinette: The Portrait of an Average Woman" from 2002 says it came from Fouquier-Tinville's papers that were searched by Courtois as well.