There are great volunteering, internship and assistant job opportunities in many fields that can polish your interrogation skills. Whether the skills that you need are for criminal interrogation or not, the important thing is to learn how to read non verbals, to maintain a same line of questioning without deviating, and to learn to get the information that you need with the least amount of input from you. This means that asking a good question is not an easy task; at least not a question good enough to get exactly the answer that you seek for.
If you are currently employed, volunteer to be part of the job hiring process. Sit with your employee and ask whether you, as a current employee, can help develop questions for future hires. Your employer will be able to help you keep the interview questions focused, which is one of the hardest skills to learn.
Understanding people is another huge step in learning how to interrogate. Salvation Army and the Goodwill industries always seek for volunteers that work directly with people who are either indigent or who seek to learn some skills themselves. Expose yourself to people as much as you can. Read their cues, ask them questions and learn what is appropriate to ask in specific situations.
Go online and meet counselors in any area. Counselors make their living out of asking people about their state of mind, their daily dynamics, etc. You may even want to work as an assistant to a counselor, or request a job shadowing experience where counselor shares with you the skills that they employ.
Since a drug/psychological counselor has patient confidentiality rules, other counseling places you could look into are weight loss programs (WW, Jenny Craig) where participants take active part in the weight loss success of their peers and serve as a source of support. They too ask them specific questions daily diet, eating behaviors, triggers, frustrations, and many other things. Ask a center manager whether you could be present during one of the consultations and explain what skills you wish to learn.
Again, keep in mind the skills that you need and regardless of what source you choose, the important thing to consider is how your questions (when put in criminal justice setting)will acquire exactly what you want to build a potential case.