Weiß jemand, wo ich das kaufen kann oder wie der offizielle „Name“ oder Typ dieser Wandhalterungen lautet?

All diy stores have or at least had an assortment of this. Think hubo gamma brico. I think they are called versterkingshoeken or stoelhoeken. Depends where you ask but if you bring a piece and go to a diy they can show you where to find it.
Bolts and plugskes can also be found there.

L-profiel or L-bracket. This looks like a specific version that came with an item though.