What source have you located for the earliest known usage of the term for the brigade?

I made the same assumption as you before I originally posted, but didn't find direct support for it in a quick check of what materials I had on hand and online. Shelby is fairly heavily followed by those studing the war in Missouri/Ark, so I had hoped an answer would surface rapidly from knowledgeable folks here. Since it hasn't so far, I'm less inclined to believe the name was used by early '63. It can be difficult to determine what is post-war tradition by the participating historians (esp. with John Newman Edwards involved), and what was actually contemporary during the war.