It all seems pretty random to me so I typically just run around. Assuming you have access to a Guild Trader and maxed extraction passive: given the price of them (on PS4 at least) you may be better off going to a lower level creature farming spot, farming raw medium material, refining them and selling the improvement mats.

I can get ~400 raw mats per hour that way. Just a tip: when you're refining, do one refine and if you only get the refined material back just leave the station and refine again until you get trait gems or improvements back and then keep refining. It seems to give better returns that way.

South-West of the map is usually a good area for me to farm in. The whole area, I run around in circles there...