By far the cheapest option is the old good Dell server PSU N750P series. Modifications to run as stand alone 12V power supply are extremely well documented on the net, it's available everywhere for less than 20 Euros or 15 GBP. Very high and stable output, designed to work 24/7, slim and less bulky than ATX, very low losses (around 7-9W), runs cold and quiet. All you need is two 6 pin cables of modular type, cut one end of each cable, one short insulated wire to bridge pins A1 and B6 on PSU to trigger auto power on, one jumper across A1 and B1, one jumper across B2 and C2 (to stop the fan from spinning full blast in emergency mode). And it just so happens that the +12 and ground output blades perfectly match with 6.3mm female crimp connectors, so adapting 6pin cables couldn't be any easier. I then use short piece of heat shrink tubing around each blade connector to provide snug insulation. 20 minutes of work tops, less than 30 quid in parts.

http://ocuk.filmsuk.com/dell_psu_adapted.jpg