Don't pay money. If you want, watch Youtube. Remember, interview prep should be about helping find your voice, being comfortable in the setting and acting professional. If you can do those and know your application extremely well, you should be fine. If you have perceived weaknesses they may ask about or interesting facts about yourself, create some bullet points, not whole canned answers. They may not ask you in the way your answer was written and it'll either sound off and totally forced, or it'll trip you up and you'll get nervous.
If they ask you something totally out of the blue, it's fine to give yourself a moment to gather your thoughts. Say something like "That's a great question. It's not something I've experienced/truly thought about extensively." Medicine is full of both introverts and extroverts, so don't shy away from who you are. Both personalities have successfully sold themselves to AdComs for years, you can too.