Most of the 'DELL's of the world have custom sized PSUs which are designed to fit in the spaces that they fit in. As such - a standard PSU will likely not fit into that same space. That leaves you with two options. Find a replacement based on the current model number and size of your PSU or purchase a new case and PSU and move the guts over to the new box.
You should be able to get the model number off of your PSU and do a quick google search. From that - you will get the exact size and see if you can find what you are looking for. Based on this - you won't have many options for a quality manufacturer (as you would with a standard PSU) - but you may be able to find something that will work.
The second option is prefered - as you...