Here are pictures of how to locate the magnets in a hard disk:
http://www.sestarragona.es/PNphpBB2-viewtopic-t-51.html -scroll down the thread. You have to separate them from the metal plate they are stuck on to with some care. What I did was hold on to the edges (of the metal part, not the magnet) with two pliers and bent it away from the magnet, which comes off pretty easily. You have to be careful because the magnet is easy to break as it is fairly fragile.
If you want to place the components on the actual cymbals these flat magnets might not be the best option due to the rotation of the cymbals - a ring-shaped magnet would be better. This is why I put mine in the pedal, and I don't really think there can be much of a difference -if any- regarding performance. All you need is a response to the pedal/top cymbal (they are mechanically connected) movement (with the magnet attached) with respect to the sensor.
As for the sensor, I can't help you out there - I had a look at Farnell.it but they also seem to be out of stock.
Maybe you can ask in the Italian forum if anyone knows about where to get it in Italy.
Or you can write to the bloke who runs bitsbox to ask if he'll be stocking that sensor again.
If it ain't broken... fix it until it is.