Food for thought. I don't know if things have changed since this was written:
http://www.easydesksoftware.com/news/news21.htm
"....So I did a search on the internet to purchase a Dell power supply, but found that no one on the internet sells one. I also found out that even if you hacked up the computer box to fit a standard ATX power supply, the wiring configuration is completely different, Compounding the pounding, Dell uses a standard ATX plug to attach to the motherboard and at this point the unknowing individual would plug in and fry the motherboard and toast the power supply. So the only choice I had was to go to Dell to get a power supply....."
Newer Dells use standard ATX power supplies. For some of the older models, that wasn't true, but there are OEM suppliers out there with power supplies that match Dell's weird pinout - googling the model number should help find one if needed.