Royal Snakes?
 

Where is the best place to find them - thanks in advance.

The witch's underground in Arlania always has them I think, but you'll have to do quite a lot of fighting to get to them, so that's not an early game option I suppose.

Marshan swamp - one of the two castles - almost guaranteed.

Often the menagerie in Greenwort (the guy with the bear quest) will have them too.