power supplies start from about £6 quid from click here but that won't fit inside a shuttle pc as these tend to have external power supplies to keep heat down and space is at a premium in shuttle pc's.
Best bet is to dissconect all non essential parts. Go for the MB, CPU+H/Sink&fan, RAM, and VGA card. Then see if it powers up. If nothing happens then its quite likly your PSU has gone, have a listen to see if you can hear the fans going. If it works then it might well by a HDD fault or somthing else. The only way to tell is the gradualy reassemble it a bit at a time.
click here a way to test the psu - note I have never done this but have seen this method mentioned in other places - short pin 14 (green) with any black (ground) wire - the psu should spin up. I would not use a paper clip if I ever were to try this - I would use a piece of properly insulated wire - I certainly would not touch anything until the mains power was disconnected.
Note beware of the dangers of electricity - it should always be off at the mains if you are poking about inside a case - if unsure of anything don't try it - get someone who knows about electricity.