You should be able to get that at your nearest Radio Shack. It may not be the exact size, but as long as it's 47uF and at least 6.3volts, it should be ok. Make sure you put it in the right way round. One lead will have a + or - sign next to it.
In your situation I don't think the magnet will cause a problem to the computer, just keep it away from the monitor (if it's a CRT), and any floppy disks. The problem might come from excessive vibrations if you turn the bass up loud.