Adding ODD to Precision 5820 Tower

I purchased a new Precision 5820 Tower and didn't realize it did not have a slim DVD drive installed (only a cover over the place for one).  I called Dell to order one, and they did not have a part number...they even told me I had to buy an external DVD drive, because an internal one was not available!  Does anyone know what part number I need so I can order it?

Thanks, Doug