Quick, cheap and dirty Pi motion detection

There a number of ways to detect motion using a Pi:

• A webcam with motion-detection software.
• Ultra-sonic sensor.
• Passive Infrared detector.

There are many easy tutorials on using passive IR detectors with the Pi, like this one, but what do you do when you want it now, there are no local parts places, and ordering can balloon with shipping/tax and involves waiting?

Reduction to a problem already solved

Swiss+Tech unit in packaging.

Enter the discount stores: Dollarama, Dollar Store, Poundland, etc. These stores are great low-priced graveyards of small electronics, that can be re-purposed to other projects. (See bigclivedotcom's YouTube channel.) I've picked all kinds of stuff in these places: webcams, Bluetooth camera shutter buttons, USB battery packs, etc, for pocket change.