Well you still have this semester + your senior year left, so there's still time. Actually, I have 3 strong recommendations that all came from relationships I built my senior year, so it's definitely doable. Is there any type of optional thesis for your major? If a thesis requires you to get an advisor and possibly a second reader, then there are two people right there that will be able to write good LoR's for you. Plus, completing a substantial research project will definitely help your chances anyway. The next best option is to try to get involved with research that fits your interest. Other than that, you could start going to office hours for classes that are relevant to your desired field of study, and have conversations about whatever interests you. Just be sure you do well in class and show enthusiasm for the material.