Visit Northampton Airport (7B2) or Plymouth (KPYM ) to pick one up and get your first stamp!
Here on the website and inside your passport.
The information is on the Participating Airports page on this website. You can also call ahead and make sure the stamp will be there for you.
You can ask an airport employee for a signature and get a photo of yourself in front of the airport's sign. It is suggested that, before departure, pilots call facilities to verify that someone will be there to stamp the passport.