If you want to, you can go observe them in the Wild near waterways along Highway 280 in San Mateo, CA. I've had pretty good luck finding them there. Dang things are hard to photograph though, they spook very easily and bolt, and tend to be way out on shallow water. It's a State Fish and Game Refuge and believe me, they are watching and are very protective of the snakes. You don't want to get caught catching them, even if it's just to take photos.

There's some awesome CA red sideds (infernalis) and other garters in that area too.

No collecting of any of them is legal and penalties are very stiff. You'll do jail time if you get caught messing with the San Fransisco garters. They are a critically endangered species. Highly illegal to possess in the US. Most of the captive snakes in Europe are descended from legally acquired animals.