How to get a song on my iPhone that is not on iTunes?

I have been wanting to get the song conquer all by behemoth and its not on iTunes & I'm trying hard to get that song but I don't know how. The album came out in 2004 and its still not on iTunes and how do I get that song & what's the easiest way to get a song that is not on itunes. Want more answers.

You can take any mp3 file you have and just select and add it to your itunes list, then copy to your phone.