Bestbuy offers SimpliSafe - Panic Button - White for $6.49 (orig. $19.99) with free shipping.

Product Description:
Take instant action against burglary and other emergency situations with this SimpliSafe panic button. When pressed, it triggers your security system’s alarm to immediately notify SimpliSafe or the police of your urgent call for help. Opt for the silent feature of this SimpliSafe panic button to discreetly signal for help without the intruder’s knowledge.

• Helps, when you're in danger.
  You can instantly trigger your alarm and dispatch police.
• Silent panic
  Allowing you to signal for help without alerting.
• Ready to work right out the box
  With pre-installed battery for long-lasting use.
• Simple installation
  Place it in the room you want to monitor and easy to add to your SimpliSafe system.
• Compatibility
  Compatible with your SimpliSafe Gen 3 home security system