Since Oxtar is now under the TCX name, maybe search around for those. The tcx site lists a replacement "dual compound" sole for the Pro2 and the other model of dirt boot. It doesn't say velco attachment though. I'll keep searching around.
I dunno... I realize it's the same vendor as someone posted above, but you'd think helmethouse would have spare parts for the sm line... but nothing listed specifically for "supermotard" boot in the back of the '08 catalog aside from the ones I listed above.