I actually borrowed it from a fellow Lateral-g member, but you can buy them from
http://www.wheelworksinc.com/mountmate.html
http://www.autowaretech.com/mountmate/
It looks like a great investment if you do alot of wheels and tires etc
Percy's also make a more generic version that doesnt actualy use a real tire.....
http://www.jegs.com/i/Percys/760/01201/10002/-1