You need them only ONCE. Then you'll be able to do Vortexx again without other crystals.
Here are the mobs that have the best drop rates (crystals can drop more rarely from other mobs, for example, technicians can drop red too):
- Engineers = Green
Operators = Red
Technicians = Blue
Superior Commando = Gold
Superior Commandos can be found in sector B, in the boss zone. And that's a pain because the mob is relmatively rare and the drop rates are not great.
You can find some of those mobs in others places, but sector A and B are by far the easiest places to farm in.