Try Amazon for laptop car supply adaptor. You will need to get the right one for your laptop. They are available on ebay but I would stay away from the £1.99 ones from China, car voltage is 12v & most laptops are 19v, so it a specialised adaptor that's required.
I found one for my brand new Acer netbook on Amazon for about £15, but my inverter works fine, so I haven't actually ordered it. Some laptops just won't charge from an inverter, some will, I guess I was lucky.
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