Looking for Interview Preparation Advice

Around ~4-5 months ago, I decided to pursue the investment banking recruitment process (IBD SA). I've been studying technical guides, taking courses, etc. to learn all the material that should be known. I think I have a pretty good grasp of things at this point. Not fantastic at it, but I think that if I spend another month or so studying I'll be at a good spot. 

Yesterday, I did my first HireVue. It went well, but only because I had unlimited time to prepare my responses. That was a wake-up call for me--I realized that I'm trash at interviewing. I can't produce clean and concise responses (to both technical and behavioral questions). I know that practice will help me, so I'm wondering if anyone has advice on how to practice generating responses on the spot. I'm thinking mock HireVue sites can be helpful. Does anyone have a site that they recommend that can ask me behavioral and IB technical questions? 

I know that a lot of people do mock interviews with friends or mentors, but I don't have anyone I can do it with. If anyone else is going through the process, is in the same boat, or whatever and wants to work together--feel free to reach out.

Any advice would be great!