hediye kartı nereden alabilirim?

Where, and whether, you can buy an iTunes store gift card will depend upon what country that you are in, they are not available in all. As you've posted in Turkish I'll assume that you are in Turkey, in which case I don't think that they are available there.

The iTunes Store on your computer's iTunes is accessible via the top middle of your library, assuming that you haven't disabled it via the Parental tab in iTunes' Preferences.