First, NEVER pay an online service or person for a mock interview. Just not worth it , those things are very scam-like, preying off the desperate.
Second, you do not need a person with big 4 experience. That does not matter at all.
Third, big 4 interview questions are pretty standard and readily available on the net. Any standard behavioral questions you can find by googling “behavioral questions” such as “Tell me about a time you worked in a team” or “tell me about a time you disagreed with a manager. THese questions are mixed with questions directed at you and your resume like “Why did you choose accounting”, “what was your favorite class” “tell me about yourself/your resume” “why audit/tax?”, And standard questions “what are your weaknesses” “why this firm” “what are your long term goals”….. ect. Spending a couple hours on google, you'll probably see all the questions you'll ever hear except for follow up questions or very specific questions about your resume/experience.
Fourth, Just because you are zero for five does not mean you are a bad interviewee. You could have easily been on the shortlist but just didn't get in for those interviews. firms have a small number of spots for a butload of students. So stay confident.
Fifth, but that doesn't mean you should not practice. You definitely should and it is something students generally don't do enough of. Mock interviews are VERY important if you are not automatically comfortable/confident with this sort of thing (few are).
Sixth, DONT just limit yourself to a handful of companies. If you are just applying to public accounting firms, chances are your missing out on some other opportunities. Cast a wide net and see what turns up. Don't just apply to 5 🙂 or 10 companies.
My recommendation: Use your career center. Also print off a list of questions you can find online (standard questions, behavioral questions, questions on glassdoor.om..” sit across the table from a friend/family member and practice looking them in the eye and answering the questions. This can be incredibly helpful and is more difficult than you think, even knowing the person and the questions beforehand.
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