Hi angelika..looks like a pretty cool design in the photo...I have had soldier flies for two years now and they seem to really stick around once they get established..
You can buy larva at pet stores or pet food suppliers but they go by a different name, but an easy way to get some going right now before you build a breeder can b easy..it worked for me.
Just put half a cantelope (skin on) in a sneeky spot out in your yard..observe it daily..you may notice a soldier fly or two hanging out on it after a day or two wait a day or so after you notice a fly and then transfer it to your soldier bin or temporary bin or
bucket as the cantelope skin is a perfect egg laying medioum for the flies and they seem to love melons and cantelope.
I have been using a super simple system in a plastic storage bin and I used. PVC pipes laying on an angle and long
enough for the pipe to protrude and the larvae to fall in a bucket....it was a little too simple for something that I would make as a
gift for someone but it worked And the bin also contains d worms that would live hqmoniously with the larvae and keep composting through the winter when the soldiers went dormant...I use the larva to supplement the diet for tilapia in an aquaponics system ....
The system in the photo looks really good as a system that could also be used indoors if the exit tube
led to a container....it looks like it is a sealed system and you wouldn't have flies hovering around the house...soldier flies are very docile though and do not bite or bother people.