I read that Sciomyzidae are also called snail-killing flies, which immediateley made me think: "I need those"!
- Are they really fit to control snail over-population in my garden? and
- if so where do i get them?
These insects live near water or marsh areas so that's where you can get them from. This is because many species of this group has larvae that are aquatic or semi-aquatic. According to wikipedia:
The adults drink dew and nectar.
And it is their larvae that either prey on or become parasites of gastropods.
In this article the following is stated:
(...) Sciomyzidae is of interest from practical viewpoints because of their potential as biocontrol agents of aquatic snails carrying diseases (e.g. schistosomiasis and fascioliasis) and terrestrial, pestiferous snails..
So the answer is yes, they might be fit to use as biological pest control, yet they may not be suitable against all species. Details of their life cycle and their biology can be found in the linked article or in this book: https://books.google.hu/books?id=vqE1AMIj-HUC&pg=PR16&lpg=PR16&dq=Sciomyzidae&source=bl&ots=pCVJJUu2cb&sig=M1F8GhR51VI1Y62htF7eX0kNpfo&hl=hu&sa=X&ei=AF0uVf_tKI3taNLhgIAL&ved=0CFgQ6AEwBzgK#v=onepage&q=Sciomyzidae&f=false