That Infectious TUNE!!!!!

You know the one I'm on about, it features heavily in the climax and throughout the whole movie. Its not part of any song and you probably can't download it anywhere but everytime I watch this movie I can't stop whistling it for days (if not weeks!). I'm here in the off-chance someone might, just MIGHT, know where I can download/purchase it? (and no its not on the soundtrack of the film).