Take a look at the taurus .44-410. I personally havent handled the revolver, but I hear it has been a fairly good shooter. Take a look at the taurus forum to see more about it. It is a revolver which shoots the .410 shell, and a .45LC, good little carryer for snakes. Also, Any .45 with the .45 shotshells could work too. I shoot them out of my .454 sometimes... dont used them for hunting anything like birds, cause they dont have the power, but for snakes they are an option.
And as mentioned above, I dont agree with Jkump about the .22 shot shells. Those things are made for small rats and mice. You put a .22 shotshell up against a snake of any size, you are going to have an angry snake. The regular old .22 is a choice though.