Belated answer, but that's actually Harvey's fence (the one you can find on his island), which was hacked back into the game after people found out that Nintendo actually had code for it (and a mermaid fence, and a white picket fence) in the game, but seemingly decided to scrap it. Which is a damn shame, because it's one of the better looking ones imo.
But regardless, currently the only way of obtaining it would be either through hacking it in yourself, or purchasing some from another player (they go for real money but for a hefty sum of NMTs as well, depending on where you look) HOWEVER, if there's one thing the star fragment trees have taught us, it's that I wouldn't invest ANY type of currency into obtaining anything hacked, because it could get patched out real quick and all your money/NMTs would be wasted.