240 volt AC fans from here:
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But I think you would be better off reconsidering your natural ventilation system.
Just as an example, if your louvres are at the side of the cupboard, and you amp's heatsinks are at the back, close to the cupboard wall, then obviously you are going to collect a lot of hot, unmoving air.
Repositioning the louvres and/or the amp might solve the problem, and of course, you would need the louvres angled upwards, rather than downwards or sideways to maximise cooling by convection alone.