Answer: Thru trade and travel!
Munk, they were seafaring and landfaring folk, the vikings. They set up trade routes thruu Russia into asia, jaunts to the new world, britain, iceland, greenland, maintained trade rouutes with the mediterranean, persians, India, etc. They could get snakes anywhere where they traded that had snakes.
He probably used sankes to horrify those folk who had never seen one, making the psychological component of this torture even greater.
It is easy to think of folk in history as barbarous/ignorant or without the means to do what we do today, and this is just not so. People have traded commodities and curiosities across great distances for quite some time. We just do it faster. Little has changed.
Keith