Something to keep in mind. Interview prep may be good for some, but you also run the risk of looking very "coached" in the interview. This could cause you to come across stiff with canned answers as your only experience with verbal communication.

My advice for what it is worth (i.e., free) is to save your money. Know what you need to know very well and BE YOURSELF in the interview. It's an interview and not a pop verbal quiz.

If you want to practice interviewing (recommended) then practice with someone who knows you well and who isn't afraid to offer constructive criticism. Someone who already knows you can better discern how you come across and better (in my opinion) identify potential strengths and weaknesses.