There are different laws for venomous in different states. I think only keeping native rattlesnakes are legal in CA.
The most famous show I know in US is the Hamburg Reptile Show in PA. Here is the web site:
http://www.hamburgreptileshow.com/vendor-list.html
In Europe, Hamm/Germany Terraristik Reptile Expo and Houten Snake Day are the best shows. I was at Houten yesterday and you can find basically any snake you want there, including king cobras
.