The idea for the air gap came from the Smart Citizen project. If you look at Google images for “smart citizen” and look carefully at the images which come up you can see there are a couple of variations of the air gap in different board versions.
The MQ sockets seem to be available on aliexpress, just search for “mq socket” although I have never ordered any.
The Figaro sockets I have only found direct from Figaro. Email : figarousa@figarosensor.com