That song was written in the 1800's by Strauss, so it must be public domain by now, which means you're free to use the composition itself however you want. You have to find a *recording* you can buy, though. I'm sure plenty of orchestras have recorded it.... you just have to find one willing to license out their recording to you. Good luck with that, hahah! Don't know where you'd start, there. If you google "royalty free also sprach zarathustra," results pop up. It's up to you to do your research to make sure they're actually royalty free, though.
I'd be careful using the song anyway, though. 2001: A Space Odyssey was so incredible, and put the song to such great use, that the piece has been linked to that movie in the public's consciousness for like fifty years now. Anytime anyone hears that song, they're going to think of 2001. Using the piece seriously is going to distract people because of that, and using the piece as a reference to 2001 or as a joke in general is so lame and played out that I cringe whenever I see it done.
Unless you're making something that's going to top 2001, I think it's best to avoid the piece altogether. So... I feel pretty safe saying that you should probably avoid the piece altogether, hahah.