While I was a mammal keeper at the zoo here in San Diego the curator of reptiles gave me a freebie couperi. Not as good of a deal as it sounds. It was free but it was also fairly old and had cataracts. The zoo had no breeding program for Drymarchon of any kind so I don't know its origins.
As far as Robs couperi I can vouch for two of them. I bought a unrelated pair of young adults from him around 2006 or 2007.
Gale