Do it. I think a professional would be best as they would be better at making the experience seem more authentic. I think it would be hard to get a friend to do a good job and make the experience seem real. My school offered mock interviews and I did it even though I think I interview OK. Was very glad to have done it. It made me think about questions that I hadn't been asked since med school interviews ("Tell me about yourself", "how do you deal with adversity?" etc...).
Check with your school and see if they would be willing to set something up. Otherwise, talk with someone in your chosen field and I'm sure you will be able to find someone who would be willing to do this for you.
