New power supplies can be purchased from:
Here is a PDF file from MacResQ on how to remove the covers to replace the power supply. Just incase you don't like my instructions :)
Here is instructions from Lance on how to replace the power supply fan if you wish to :)
You must first have removed the front cover & top covers and the left side panel.
Here's a picture of the side with the cover removed. We'll be dealing with the power supply in this section.
There are 3 screws that need to be removed. 2 on the back of the case. 1 located just above the vent and the other just below the outlets. The 3rd is inside the case deep behind the power supply cables. Take care not to drop it or you will be hunting for awhile :)
With the screws removed you can now lift out the supply. I find it easier to do with the case lying on its right side. First move it to the front of the case about 1/4" as there is a lip it is under in the rear. Then lift up taking care not to damage the cables (white arrow). They will get pinched as it is a tight fit, so do this slowly!
Now unplug the 3 cables. 2 look like HD power supply cables with one being extra large with more wires and the 3rd has a ribbon cable.
With the supply removed you can either replace it or clean it with some dust remover spray. I got this can from Radio Shack. Just spray the air in the vent and fan opening.
If you wish to go further you can open the supply by removing the 4 screws (red arrows) located around the supply. When reinserting the pieces make sure the tabs (black arrows) are seated properly.
WARNING! there are some capacitors inside the power supply which could still hold a charge even after power has been removed. The capacitors are the tall cylinder devices. try not to touch the leads on them just incase.
There is a fuse located just behind the outlets labled F6.3AH 250V. You might try checking this fuse if your removing the supply because it is defective?
Reinstall in reverse order. When reinstalling the power supply in the case, be carefull of the cables again because you don't want to cut any of those!