What is this called?

[nimhed]

and where can one find it?

 

[BirgitS]

Sheet anchor: https://www.hornbach.se/s/skivkankare

 

[Turbo_Tail]

Isn't it a "spring toggle," which might be called fjäderlåsning in Swedish?

https://www.screwfix.com/c/screws-nails-fixings/spring-toggle/cat840118

 

[z_bumbi]

Plaster anchor and any hardware store.
May have additional names for the specific type of anchor but it seems to vary depending on the manufacturer.