If you know where any garters can be found, my suggestion would be to build yourself a trap. I have a thread where I show how to build one.

However, you may want to check local wildlife regulations to see what you can and cannot do for catching snakes. For example, in my native Georgia, it is illegal to capture, keep, transport, buy, or sell Thamnophis sirtalis without a license. So if I ever needed to go back home for some reason, I'd have to do something with my snakes. Probably find a science teacher or snake lover who would take them off my hands.

But like Thunder Girl says, your best bet might be to find someone on this forum who is willing to part with some of their babies.