any trick on the purple Vials in Zombie modes

I have like 60 of the blue, and a lot of gold, but i do not often see the purple, and i am kinda stuck in research..

Are there any trick to finding these, or should i just keep hammering on Zombies to get the sample bags?

Tried a few of the No mans land, had a few in the start, but nothing since. again the occurance of Gold are WAY higher.

i am trying to get the perk that lets me search the hidden compartment in safes.