There's a nice guide to rolling your own on
Ian Stedman's web page:
http://www.ianstedman.co.uk/Amiga/amiga.htmlClick on "Amiga Power Supply Guide."
I used an ATX supply and a spare power plug & cable
that I got off of e-bay for a few dollars. I mounted
an old-fashioned terminal strip onto the
side of the power supply and soldered to that. If you
can find a supply with a real rocker switch on the back,
you can avoid adding extra circuitry for the on-off switch.
Larry