Ebay offers Google Nest Temperature Sensor for $20 (reg $39) with free shipping.

Product Description:
Meet the Nest Temperature Sensor.
Most homes aren't the same temperature in every room. With the Nest Temperature Sensor, you can let your Nest thermostat know which room should be a certain temperature at a certain time of day. Just place it on a wall or shelf and get the right temperature, right where you want it.

• Dimensions (Overall): 1.9 Inches (H) x 1.9 Inches (W) x .8 Inches (D)
• Weight: .1 Pounds
• Features: Voice Control
• Industry or Government Certifications: No Certifications
• HVAC Compatibility: Electric, Gas, Central AC, Oil
• Compatible With: Nest App
• Material: Plastic
• Battery: 1 CR2023 Lithium, Required, Included