First is who to get a LOR from. Each school has different requirements so check to see exactly what they want. I would recommend 2 from PTs and at least 1 from a professor (preferably a science prof). Other potential people that might be good are former employers and your major adviser.
Pick people that know you and you have had good experience with. Getting a LOR from someone who doesn't really like you is extremely counter productive. They should be able to speak about your potential as PT, your character, work ethic, and/or your academic ability. Maybe they can't do all of these things but that is why you get multiple recommenders.
If you don't already have the relationships in place to get a good set of LOR, then start making an effort to get them. Which maybe means working especially hard in a class and going consistently to that prof's office hours. I had a class that I enjoyed and really like the prof. I decided he would be a great reference so I went to more office hours, worked hard for every point in that class (sometimes I might not have worried as much about about a half point here and there), and didn't miss any classes. So yea I am a decent student, I'm pretty smart, I work pretty hard, but I also showed my prof I was all of these things. You can't just show up once you have to be consistent.
Once you identified your people get them a copy of your cover letter and resume. Maybe even talk to them more if they need clarification.
I had one school ask for one from a introductory or general science course (basically physics, chem, bio, and they might have counted anat and/or physiology). I went to a large state school with 150+ in those classes and they were several years ago, there is no way I could get a LOR that would do me any good. Getting a letter from someone who barely knows who you are isn't going to help your app. Instead I got letters from 2 profs that taught upper division classes (with class sizes of 35 people and 3 people) in physiology classes. I am wait listed at that school so they didn't throw out my app.
