And if you're getting them from a commercial source, they are flash frozen. In other words, it happens instantly. They don't feel a thing, or suffer at all.

It's not absolutely necessary to feed them mice. I had a pair of Oregon red spotted garters that lived about 20 years and they never ate a single mouse their entire lives. I wouldn't feed them food that has been dried though. Frozen/thawed fish and earthworms will do just fine but they need the whole fish. Guts, skin, bones, everything. Most pet stores with an extensive aquarium section sell frozen silversides. They are often small enough that you can feed them whole, or you you can cut them into sections. They are a very safe fish to feed to your snakes. Thaw them thoroughly first of course.