I've got an independent, local diecast store - that, I'm sure has/had those (Larger vipers).
Here's the link:
http://www.skidmarksdiecast.com/
He doesn't do a formal internet/mail order business - but I'd be willing to bet he would get them to you/help you out if you can't find them anywhere else. (His prices are good/reasonable on Corgi, Dragon and other stuff I'm familiar with.)