Well, the one in the video looks like a solenoid with the plunger removed.
They're rather easy to find, and if you're mechanically inclined at all you'll know where hundreds are!
In cars alone:
Pretty much every emissions / vacuum / boost solenoid to control air/vacuum/boost
Power door lock actuators (some may be gear driven, but most are simple push-pull solenoids)
Air conditioning solenoids (in cars)
You can probably buy 20 of them at a junkyard for under $10 bucks, since it doesn't matter if the housing or nipples are broken off of them.
Any equipment that has the ability to turn on and off, let's say.. water or fluid as well.
A solenoid is an electromagnet wrapped in a metal can, with a hollow tube in the middle. The plunger is a steel shaft that sits in the tube, partially spaced out with a small spring.
When electricity is added, the plunger shaft gets sucked into the can, pulling the linkage it's connected to in with it.
Pull the plunger out, and put a bolt with a nut on one side, as the OP did (at least it looks like).
You can epoxy the hall effect sensor on the head of the bolt through the electromagnet.