Sandbox sells these boards on ebay for $22.95USD.
Single Channel Gateways are not LoRaWAN compliant, are not supported by The Things Network and should only be used for testing and development purposes.
Sandbox has a GitHub repository at the link below, however the firmware appears to be version 0.8.1 (25/01/2018) and is out of date:
This hardware can also use the generic ESP8266 Single Channel LoRaWAN Gateway code maintained by Maarten Westenberg. At the time of writing, this is version 5.0.6 (12/02/2018)