Hello @Lorry ,
Any sensor that you can see on the PurpleAir Map is a public sensor and is queryable. The sensor index, which you need to query a sensor, can be found in the URL when clicked on a sensor on the map (see image below for reference).
Please note that the index will be the string of numbers directly after “select=” and before the # sign.
If you need to find large amounts of sensor indexes, you might be interested in performing a bounding box API call.