If you are a foreigner looking for a long term (1 Month+) place to stay in Berlin, Wunderflats.com could be a good option to find a house.
It is like Airbnb but for long term rentals. I used it to find the place I am currently staying in.
Units on the website could be a bit pricey compared to what’s on the market. But you don’t need Schufa (Credit score report), or crazy 3 months deposits that you will get back 3 months after leaving the place.
At least this was my experience. Give it a try if you are coming to Berlin and looking for a place to stay.