Be prepared that the snake might not eat the mice, even if you feed many to make up the difference. Some snakes can be picky eaters!
That is great you don't feed live. Yes, they eat live in the wild. But in the wild, a terrified prey animal isn't being dropped into a very enclosed space, to be cornered with no escape. It turns the mice or rats into fighters, potentially injuring the snake.
You can do a couple of things if you cannot find prekilled. You can buy the live then kill yourself. There are methods that are more humane than others out there, like cervical dislocation. Or you can restrain the rat in a way that it cannot harm the snake until it is securely wrapped by the snake and cannot bite. This usually involves holding the rat with a set of tons by the scruff, then offering the snake the rat butt first. Keep holding the scruff to control the rat's head as the snake wraps it, and then only let go when you feel it is safe.
