I used this speaker in a Firepower and it works well. Some people like this speaker better than another machine with the stock speaker. http://www.marcospecialties.com/pinball-parts/5555-13961-00

If you don't want to order one from the states, you can find one with similar specifications. When I got my the machine I used this speaker in, it had a small speaker pulled from a radio or something so I suspect almost any speaker you have laying around will do in a pinch.

Just make sure you match the resistance of the original (8 ohm I think). If not you'll need to add a resistor. Check around the forum for a discussion if you go that route. Size match is useful of course so it matches the screw holes and the hole in the cabinet.