XPS 8930, replacement PSU

Your XPS 8930 was originally shipped with a 460w power supply.  You can purchase a 460w OEM replacement directly from Dell  https://www.dell.com/en-ie/shop/dell-460w-power-supply/apd/cpl-0ffd6/computer-chassis-components.  Or, you can upgrade to a higher 850w PSU option with Dell part number 9XG5C https://www.dell.com/en-us/shop/dell-850w-power-supply/apd/9xg5c/computer-chassis-components

Alternatively, since your XPS uses a standard ATX power supply.  Therefore, you can source PSU from third party makers in well known brands.  For example, Corsair RM850e, NZXT C850, Gigabyte UD850GM. 

To add to chino’s good answer, 8930 has a clamshell psu compartment therefore a third party ATX psu better not exceed the depth of Dell oem psu.  The ATX psu dimensions only specify length x width. Different ATX psu may come with different depth and if too long may not fit 8930 chassis, which is also true in Aurora R7/8 that uses same skeleton metal case.

from official Dell mod Chris M

here are Dell oem psu DP/N

The XPS 8930 shipped with the following PSU =
9XG5C 850W Huntkey
PK61K 460W Delta
WC1T4 460W Huntkey