Dale, I believe they are called Uni-Drives. Altoona Pipe and Steel next door to us uses them on their rail car repairs. I could try to get you a contact with them if you like.

We ended up using steel flat head screws as they looked the most like our original rivets.

I would encourage people to try hot riveting. We have found that with four people the job goes smoothly - one to heat the rivet with an acetylene torch, one to insert the rivet in the hole, one to work the rivet gun, and one to buck the rivet. (Being the least competent, I have the last chore.) It is work and takes practice, but when you are done you have a structurally sound product and the satisfaction of knowing that you haven't compromised the restoration process.