I'm with Lurch on this one, I would go witha TFX PSU tho.
The Amiga 3000 PSU is 110x63x175mm (WxHxL) and the TFX standard is 100×63.5×125 mm (W×H×D) so they are pretty close in size (lenght don't matter).
be quiet! makes some quality PSU's that will last for a long time with really low ripple and as a bonus they are quite quiet also
At loads the Amiga 3000 pulls i'd bet the "be quiet!" PSU's will be well under 20dB
