Any involvement with ASDA or access to dental students if there's a school nearby? Those were my best ones. They're not pros, but better than someone not involved with dental school at all. Generic interview skills are good. But with students, they shared info and tips about not only their school, but other schools where they interviewed and a variety of interview styles. A few had sit in on interviews as well. They were certainly open with criticism too. None of this 'great job, you'll do fine' stuff you might get from family and friends. I had 1 recorded and another videoed too. Yikes. That reveals a lot. I just did some mock interviews with pre-dents at my school; it was a lot of fun and good to share my own experiences.
I wouldn't go out of your way to pay for services. Does your school offer anything? Like your pre-health people? If you're really uneasy about it, start by just asking yourself some question and saying it out loud. You might know why you want to be a dentist in your head. But can you convey that to someone who asks (without sounding rehearsed or not thought out)?