There are various type of door stoppers depending on the situation.
Some are non magnetic like this


for the magnetic ones


Then there are some that are floor conceal type like this.


Can get them from the following places. Else can also try your nearby hardware stores.

HERE

HERE

You could use those Restricted Child Safety Friction Stay Window Hinges. Looks better IMHO


These restricted child safety friction stay window hinges only allow the window to open a small distance.
A button can be pressed on either one hinge or both hinges (depending on the brand) to release the window the full distance.
The buttons click back out automatically when the window is closed.

You could check with your window supplier.

These Restricted Child Safety Friction Stay are commonly used in high rise building. Have been seeing them in hotels rooms that I have stayed in before. So, should be easily available.
There is also another type which I noticed being used in my office's window. It basically uses the normal friction stay plus another device installed nearer the bottom of the window which will control the window's opening.
Both functions the same.