The devices retail for $15 to $60 USD and are widely available. They can either be battery-operated or AC powered (with or without a battery backup). Battery lifetimes have been increasing as the technology has developed and certain battery-powered devices now advertise a battery lifetime of over 6 years.
They can be purchased at your local hardware store and at most large department stores. They can be purchased online as well. It is up to the homeowner as to what model suits their lifestyle.