>>939300You can diy one pretty easily using a large transformer and a battery. Just remove one of the windings on the transformer and expose the core. Even a small one can be pretty powerful.
https://www.youtube.com/watch?v=wzXRFp0DDrUJust remember that those wires for the coils are coated in a very thin enamel. It needs to be protected from scratches (unlike the video above). One scratch can short them out. Use fingernail polish or epoxy to help fix scratches. You'd also need to make a water proof container for it. The one in the video above is the best you can use in reference to how its primary and secondary coils are put on. Some transformers have the primary and secondary coils wrapped on on top of the other making it a little more difficult to get them apart. Always use the coil that has the larger wire for a electromagnet.
You can also use your own magnet wire to make your own coil for use with an old transformer,
http://www.instructables.com/id/Strong-Electromagnet-From-An-Old-Transformer/?ALLSTEPSRemember, the more amps used the stronger the magnet. The more wraps of wire the stronger the magnet. The closer the wraps are to the metal you wrap it around the stronger the magnet. Thicker wire allows more amps, but smaller wire allows more wraps. Thus, there's a balance between everything for optimal performance.
Even a tiny battery will give you lots of power,
https://www.youtube.com/watch?v=SGoOu8cPmeMIf you used a car battery, I'd recommend some sort of resistor so to prevent the wires from burning out. Just use a resistor that allows x amount of watts like a 12-volt incandescent light bulb in whatever wattage would make a weak electromagnet. More watts for the resistor means more power for the electromagnet. Even a tiny transformer can make a seriously powerful electromagnet. The MOTs in the vids could make one that lifts 100s of pounds easily.
https://www.youtube.com/watch?v=6Xo-uaEY2XU 
