It's pretty simple and there's nothing wrong with asking. Plenty of people don't make a habit of saving job descriptions and as you've discovered sometimes companies take them offline before candidates get the chance to. The only way for you to come across as unprepared is to wait too long to ask about this or to walk into the interview having forgotten the details in the job description. Simply say the following in an email:
While preparing for my interview of the Xth I wanted to review the job description but noticed it has disappeared from the site. Could you send me a copy?
You can embellish the request if you like but you really don't have to overcomplicate it. If you have yet to confirm the interview time you'd do that in the same mail and open with that instead.