Where to get them: drugs stores, hardware stores, construction markets and sometimes the grocery store.
How much: costs anywhere from €2.00 to €10.00.
Installation difficultly level: super easy.
It will keep out: mosquitoes, bees, spiders, flies and most other creepy crawlies. You can get them in different sizes so measure your window first. I didn't buy this one, we had it left over from last year.
There are two parts to the screen. A Velcro tape to go around the inside of the window and the screen. It's important to put the Velcro tape on the inside, just outside the black rubber seal. Once the tape is applied the outside of the window gently press the screen onto the Velcro. It's important not to stretch it too tight or leave it too loose.
If your screen is too big you can cut it to fit. I usually only cut after it's up because if it's too small the whole thing is wasted. This is right before I trimmed off the excess netting.
And that is how Germans keep mosquitoes and other unwanted guests out of their houses during the Summer.