Any standard 24/4 pin ATX psu w is compatible with 990 tower MT motherboard. If it is 990 DT since you mentioned desktop, it uses nonstandard form factor psu.  You can still install a standard ATX psu either outside the case or inside if you remove the DVD drive bay. [IMAGE]  
I have a Dell optiplex 990. SMPS required urgent. DELL SERVICE Delhi India, told this is purchased in Kuwait.  Spare part not available. I need to repair. My location in Kochi Kerala State, India.
SMPS FOR OPTIPLEX 990 Desktop Dell