Black mosquito larvaes are best food if from the own garden or balcony in water without fish!
At the beginning of my live with Paros I was also very anxious about catching living food. I collect it just in my own garden or from the balcony. My dragonfly larvaes I got with bought plants.
This spring I tried to get black mosquito larvaes in a still water in the forest, as Bernd recommended, and there the first two insects I caught were dragonfly larvaes … so I stopped that project 😉 but in my “homebread” mosquito larvaes water I found never a dragonfly larvae, they are too far away from any bigger water were dragonflies live.
Microworms are fine, but containing high amount of grease, so it is not ideal as an everyday food for adult fish…