Image via Heritage Auction Galleries.
The banner was available for purchase by theatre owners through the pressbook. It cost $1.75. This particular one was sold through Heritage Auctions of Dallas back in March 2008 for a sum of $567.63 (USD).
The original 1938 pressbook, p.21...
Pressbook scans courtesy of the Michael Filippello Collection.
A similar but larger Snow White valance was advertised in the January 29, 1938 issue of The Film Daily (p.4). Dimensions 39" x 90". Price $2.95 each. Available from Art Flag, Inc. in NYC.