http://www.newegg.com/Product/Product.aspx?Item=N82E16817139003 or any good quality power supply
Dell uses custom OEM motherboards, but they have been using standard ATX power supplies for a long time. Just make sure the power supply has either a (standard) 24 pin or a 20 plus 4 pin for the motherboard and a 4 pin Molex for CPU power and whatever other power connections you need