Magnetic contact switch (door sensor)
We can get 30 units in Australia from Adafruit. If you order today, we can dispatch this stock between Dec 14, 2019 - Dec 17, 2019.
This sensor is essentially a reed switch, encased in an ABS plastic shell. Normally the reed is 'open' (no connection between the two wires). The other half is a magnet. When the magnet is less than 13mm (0.5") away, the reed switch closes. They're often used to detect when a door or drawer is open, which is why they have mounting tabs and screws. You can also pick up some double-sided foam tape from a hardware store to mount these, that works well without needing screws.