The acappella version of M&S is on the rerelease of HiFi and The Last Stroll, which came out back in May. I have a clip of the recording (not full length, sorry) at my website Walternative.com. I also have clips of songs from all eight of his solo albums as well as his albums with The Malibooz and The Brooklyn Cowboys.
I am so jealous that you got to see him live and meet him. (He's such a sweetheart.) I've been a huge fan of his for six years now but live in California and can't afford to travel.
Here's a Windows Media vid of Walter performing Magnet at last year's Nashville Connections show:
Nashville Connections