Ebay offers 2nd-Gen. Ring Alarm Contact Sensor for $14.99 (reg $19.99) with free shipping.

Product Description:
Expand your Ring Alarm system to secure each door and window in your home, and get alerts as soon as they open. Ring Alarm Contact Sensors mount to any door or window frame in your home for easy, DIY installation. Connect them to your Ring Alarm and receive mobile alerts from the Ring App so you’ll always be the first to know when something’s ajar. *Requires Ring Alarm Security Kit (Gen 2).

• Get instant mobile alerts when doors and windows open in your home
• Now smaller to enable increased placement options
• Easy installation with no tools required
• Mounts to door and window frames
• Includes two 3V CR2032 batteries
• Requires the Ring Alarm Base Station