New Problem Dell 546 SLim Case Need power supply

Anyone know where I can get a 300 to 400 watt power supply for my Dell 546s SLim Case. I cannot seem to find any???? If I want to be ghetto can I run a EXTERNAL power supply????

lost cause I guess. It all right I did not buy the PC for gaming but I thought it would be cool to get a PSU and new Video card for less than a console. I can still play BATMAN: Arkham Asylum that is all that matters, THANX FOR ALL THE HELP GUYS FROM MY OTHER QUESTIONS!!!!!!!!!!!!!!!!!!!!!!!!!!