I mean, ideally, you'd like to work in a lab where you're interested in the research, but if not, its no big deal. As far as finding the job/opening, I was able to do this at my undergrad lab and my PI hired me on the spot for one full year so it was pretty easy. Its one of those jobs that is a job, but not super formal, but can sound very elegant during a medical school interview, which is why I'm a big fan of it. Did you go to a research-based university? If so, did you do research in a lab? If yes to the two questions before, start with your own lab!
If you're joining a new lab for the first time, it can be tricky. PI's think about money and funding constantly (not their fault...entirely). So, a lab won't just hire you with zero experience and pay you. PI's (its awful) sometimes try to trap students as permanent volunteers making you work full time without paying you - don't fall into that trap. Its fine to be like "Hi I'm a graduate of X university where I majored in Y. My goal is to be in medical school in two years. During this time I'm really interested in broadening my research experience and want to work with you. I still need to perfect my techniques, so I'd like to start somewhere in the lab and, ultimately, if possible, work my way up to a paid research assistant/lab technician position in about 2 to 3 months. I'm highly motivated and hard working, willing to read and practice however long it takes, and then being to do some meaningful research!"