Dell should provide you a new one, free of charge, or for a very minimal charge
nevertheless, you need to contact Dell as i'm guessing that machine is not using a standard size unit, and only Dell can provide the unit you need (or you could go straight to their supplier, PC Power & Cooling)
I have just purchased a Dell 4300s and I need to purchase a power supply. If you have any recomended sites or address where I could find one that would be great. Nothing really powerful, just one that will work.