Roadkill, the snake we use is western diamondback rattlesnake. The watersnake that you mention would probably work very well, but I don't know a source.
When I was a teenager in Indiana I was down in the Wabash valley frog hunting with a couple of buddies. We were in the middle of a small pool of water that had survived a dry summer when all at once I spotted about 15 snakes in the pool. When I announced the situation the water boiled with 3 guys getting out in a rush. Normally I'm not afraid of snakes, but when the rush started I didn't want to be last. (Mind you I wasn't afraid, I just didn't want to be last.) There were probably between 30 and 40 snakes in that pool which was only about 10' X 20'. Guess it was the dry weather.