According to artist Tom Whalen, who shared his art work (below) on Instagram yesterday, this limited edition print was used "as part of an exclusive walmart promotion in april" but there's no record of it anywhere.
After much sleuthing The Brick Fan has learned that the offer was only run in five Walmarts across the USA. Our brothers at Brick Fanatics have posted details of the five stores across the US that held a minifigure trading event (no date given) which had the poster as a reward. As an extra incentive the five selected stores had LEGO Star Wars displays provided by local LEGO User Groups.
One promotion that was run on May 4th was an in-store giveaway for a sticker sheet that had the five retro minifigures and the 20th Anniversary logo. Typical of the secondary market, it is heftily marked up on eBay.com for an average of $15, plus postage.