Colin,
I took Charlie's recommendation and contacted Blyler Rivet Supply at
800.325.3147. I think I paid $173.00 plus shipping for it.
I also bought a box of 100 Olympic RV6607-5-5W rivets at Rivets Plus
http://www.rivetsplus.com/ for $22.60, about twice as many as I'll need but
better to have more than less.
Glyn