Congrats on your Japan trip! I’m jealous! If you make a point to use Japanese while you’re there, you’ll definitely see your skills grow in those three weeks (At least that’s how I felt on my trips there).
I think that hiring an iTalki tutor would be helpful, but if you’ve never spoken be prepared for a little bit of a challenge. It’s a much different skillset than just listening. However, if you’re dedicated you can probably pick up on some basic patterns in a month’s time. I think you can probably find a conversation practice partner for around $10 USD per session. I would message them letting them know exactly what you need help with and make sure to have a concrete plan before going into your first lesson.
On the free side of things, もしもしゆうすけ has some fantastic videos filled with everyday dialogue such as the conbini, fast food, etc. I watched each of them multiple times over before my last trip, and they were helpful to me. They help you pick up on the style (and speed. It’s a lot faster in real life.) of Japanese you’ll be hearing. I’ll link a few of my favorites but feel free to explore his channel!
Conbini: https://www.youtube.com/watch?v=4NjSKPPFaEU
Fast Food: https://www.youtube.com/watch?v=lLjPNTpxtjg&t=613s
Drug Store: https://www.youtube.com/watch?v=fzVKlyZeibo
There are a lot more helpful videos on his channel, so definitely go through and watch!
I hope you have an amazing trip!