In my experience, I’ve received them randomly from Portkeys. I’ve also seen them (less frequently) on the ground during some kind of event (Brilliant Event, Community Day, etc.). They still seem to be somewhat rare.
Found two on the ground. About three weeks apart. The last one was about 4 days ago.
UPDATE 2019/10/17: We find them all over the place now. Not so rare anymore.
I finally found 2 this morning. They were in the portkey with the big hand
I’ve only ever gotten them from portkeys. I think it is a portkey only ingredient.