All it needs is 12v at around 5A total (you probably will draw less than 4A, but some spare capacity is good).
The VRM will handle higher voltages, so if you can trim the PSU to 13v or 14v, it will reduce the amperage in - the 5A fuse is a bit undersized if you have significant voltage droop, and will blow due to the higher amperage pulled if the voltage drops significantly (things will keep running, the VRM will still work, it just pulls more current).
A typical computer power supply won't be very efficient supplying only 40W, but a lot of the "standalone 12v PSUs" on eBay won't supply more than 2A on the 12v rail, which won't work.
Something like this might be fine:
http://www.ebay.com/itm/Emacs-P1A-6250P-250W-Power-Supply-3x-Molex-Connectors-only-/111069759593?pt=PCA_UPS&hash=item19dc460869Otherwise, just plug it into your PC as noted.
