I would like to get a small, nonvenomous snake...what kind?

I have a 20 gallon gl aquarium...I was thinking a corn snake maybe...can you tell me some kinds of small, nonvenomous snakes? What supplies should I get? What can I feed them?(I REALLY REALLY don't want to feed them mice or rats-I have a pet fancy mouse right now...) Are there any alternates to feeding them rodents? Where should I purchase a healthy one from? What is the average life expectancy? Thank you.