Ducky -- when I searched the web for power supplies, I found a whole lot for electronic hobbyists, but found that it was quite hard to find ones designed for computers, with ATX connectors, etc. Also (you may know this) the AC adaptor brick is only the first part of the picture. It delivers 12V DC (for example), but you then need a DC-DC converter board that goes inside the case and converts that 12V to the various voltages required by a computer mainboard and drives.
d_kay -- I did find a couple of manufacturers, though not as cheap as I would like.
makes a relatively expensive one for minii-itx and they are apparently working on a 100W external supply for beefier computers.
See my post here:
for info about the external 55W power supply I ended up buying