I think there wasn't a lot of demand for such disks and so they weren't manufactured. You'll either have to go with the 4GB RW DVD disks or just buy some dual layers. Disks aren't that expensive these days thankfully
I always used to get mine from here mate - DUAL LAYER DVD used to pick them up from either Wardour Street or Wembley, although they do deliver, for a charge.
With standard dual layer disks being so cheap, all I can think is the manufacture must of focused all of it's resources on them.
He's looking for DVD+-DL Re-writables. Dual Layer rewritables are very rare or don't exist as far as I'm aware.