Mid/Side encoder/decoder
https://www.youtube.com/watch?v=N6w5Og0n7kM&feature=player_embedded
https://www.youtube.com/watch?v=V7g6R4JUsWs&feature=player_embedded
https://www.muffwiggler.com/forum/viewtopic.php?t=142881
Available as an assembled Module and as a DIY project.
No info about availability.
$90