Off Amazon - the exact one I bought isn't available anymore but they're all about the same.
Here's a cheap one:

https://www.amazon.com/AmazonBasics-Chest-Mount-Harness-cameras/dp/B01D3I8A7A/

The official GP version is only a couple bucks more:

https://www.amazon.com/GoPro-Junior-Chest-Mount-Official/dp/B00F19Q2Y8/