How long is the sensor probe's cable? For a composting operation a probe of around 50-100cm (20-40") would be perfect to bury inside the pile. Are these sold separately?

The sensor cable for the probe is 2 meters long (6.56 feet). They are fixed to the logger and are not sold on their own. If the probe was to become damaged the logger would likely need to be sent back to the manufacturer for repair or be replaced entirely.