turkey3_scratch :
Dell and other prebuilt computers have poor quality power supplies not suited for running a $330 graphics card. I recommend you buy a new power supply to ensure that you have a safe one for your new expensive card. A Tier List can be found
here and shows what power supplies are good, and what ones are bad.
dude, the OP is asking for a PSU.
as for the question, BTX PSU's are not common, and the ones that exist aren't good quality. I recommend getting a new case, unless your motherboard is some propaitary dell form factory.