I need to know what exactly means natural flat. And where can I find natural flat headphones with the impedance of 40 ohms. (32 to 60).
Flat means that it reproduces evenly and consistently across the frequency spectrum. Also, I don't care what any manufacturer says, flat headphones don't exist. There are some that are better than others, but there is always a compromise somewhere.
You may want to ammend your question to describe where you intend to use your headphones, and for what purpose. People here can give you good suggestions, but application matters.
Flat response just means that the output of the headphones is equal at all frequencies.
In order for headphones to have a flat response they usually need to be open backed, which makes them not suitable for monitoring with live microphones.
There is no such thing as a flat response for loudspeakers/headphones but some get close.
Choosing headphones is very personal, I am a Beyerdynamic fan, others Sennheiser or Sony etc.
The best thing is to try lots.
Let us know what you choose in the end.
Here's the Flattest headphones I've ever seen, though it's not very practical for this kind of work and not mention the price.