I'll take a wild stab at this. I would say that there may be several dozen people breeding Triturus. But I would bet there are fewer than a dozen that have ever sent a significant number of offspring to other people. At any particular time, the number of US caudate breeders with animals "available" is usually zero. When animals do become available, they usually go to people who have requested them months or years in advance.
I did breed and sell/trade T. karelinii once, but had to get rid of the group during a downsizing.
I would welcome any input from anyone that has a more authoritative answer.