LED- I used a 10mm diffused white LED. 15-50 Cents. You can take an LED pretty much any old electronic device or buy them online. Radio shack sells them for an arm and a leg.
Coin Cell Battery- I used the CR2032. Very Common. You can get them at radio shack or online. Even Big Grocery Stores have them. They cost any where from 20 cents a piece online to close to $5 dollars at radioshack.
Magnetic Switch- I used one that I took off of an old sliding door. It was used for a security system. You can use either a switch that is always on
except for in the presence of a magnetic field or one that is always off
except for in the presence of a magnetic field. I used one that was always off. You can buy these online, at radioshack or at your local hardware store. ( Please Shop local, no big box stores. Big Box stores are evil!!!) They cost about $3.
Electrical tape- About 20cm. You could even use duct tape.
Wire- ~15 CM of wire. Any will do. Take it out of an old car.
A desk drawer, glove box etc.
Magnet- I would recommend a rare earth magnet. They are really strong, and pretty cheap. You can get them online or cannibalize them out of old toys, hard drive or any number of electronics. They only cost pennies for small ones. There are some really big ones that cost a lot though. ($100s of dollars!!)Tools
Wire Cutters- I used my MAKE: Warranty Voider - Leatherman "Squirt" E4 (electronics version)
Pocket multitool. (BTW this is the best Mulitool I have ever owned!!!)
Screw Driver- For wrapping wire around the terminal screws.