I checked the wiki and CTRL+F'd the drop charts, sorry if this is answered somewhere else.
How do you obtain a D Photon Core?
edit: forgot Spuz's mag guide exists. It's an OF drop in Ultimate for certain ID's for anyone else who might be curious. Not sure how I missed it on the drop chart.
Try whitelisting the PSO directory with your antivirus. I had to do that the other day after a fresh install. You can find the directory at:
C:\Users\username\EphineaPSO