Thank you for posting this. It seems it's easy to find friends to do interviews with, but it is not. When I was looking for a job, following are the services I have found. Some of these may help (not affiliated with either):
1. Pramp.com: Practice technical interviews
Downside: Not very effective if you find an interviewer that is casual. Conducting interviews is a skill. The interviewer needs to have a good command over the problem and its nuances.