Try
APC, the UPS and power supply people. I have a universal charger from them for my laptop which covers currents from... lessee... 15 to 20 volts. It's got you covered.
Plus, the model I've got has different adaptors for different model computers, as well as for different plugs -- car, airplane, and wall socket. I've used the car plug... that was cool, surfing the 'net while my wife was driving us through Illinois
It's around US$60, IIRC; don't know what the prices are like where you are, but they show the UK as a place where you can buy them.
BTW, are you sure it's the power supply, and not the battery? I've had both go bad, and the symptoms are remarkably similar.