I wanted to add some thoughts about the master's degree, as that was the route that I had taken. What the master's program adds is graduate coursework in research methodology and statistics, time working in a research lab as an RA, and the opportunity to do an empirical master's thesis. As
@MiniLop said above, none of these things are strictly necessary, especially since you can get the time working in a lab and opportunity to do guided research (via poster presentations or even publications) as an RA anyway. However, whatever coursework you do during the master's program may not transfer to your eventual Ph.D. program; typically, there is a cap on how many units you can transfer, and there are classes that you will not be allowed to transfer at all. Thus, you're spending a lot of money -- last I checked at my alma mater, the tuition and fees add up to $25,000 for a two-year (full-time) program -- to repeat some or most of the courses, even if you're able to transfer your master's thesis. Master's programs tend to be unfunded.
When you find that RA position, paid or not, learn the ropes but also think about how you can apply the skills that you are learning to your research interest. Depending on what supports are in place at the lab in which you work, you might be able to work with other RAs, postdoctoral fellows, or even the PI to develop a presentation or eventual publication, and that is the sort of experience that is more directly applicable.
If you want to look at paid RA positions, then common places to find these postings aside from individual university websites, Indeed, Glassdoor, and plain old networking (e.g., ask your former professors if they know anyone who is looking for an RA):
Post-bacc RA Opportunities (pro-tip: Options > Subscribe via email)
Post-graduate Research Jobs
If you know what research area you want to pursue in your graduate studies (e.g., schizophrenia), then subscribing to the listserv for that specialty area may yield more relevant job postings and (perhaps more importantly!) up-to-date discussions about what's going on in the field. What are your interests?