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A generic "universal" car laptop PSU should work fine - eg. this is the first i found on ebay, it has a 15V setting and multiple different connector types.
Just be sure to fit the correct plug and check it it the correct way around - the wrong way will reverse the polarity & likely wreck the amp.
I'd either check it with a multimeter to be sure the inner is positive, then tape it on the cable, or have a suitable plug soldered on instead of the swappable tips.