In the US, call Dell Spare Parts (outside the US, call your local Dell sales office). 1-800-357-3355  
I just bought a Dell XPS 15 9520.  I already have a brand new M.2 NVMe that I want to add as a second NVMe into my XPS. I have opened the laptop and discovered that there is not a second heatsink or a screw inside the laptop for adding a second NVMe drive. Does Dell sell a heatsink and screw for the second NVMe slot?  If so, how do I go about ordering them? Thank you. Tiny Tech  
Dell XPS 15 9520 - Second NVMe - Heatsink and M2x2 Screw