Hi.
1. There is nothing wrong with paying for material. in my experience physiotherapy courses and textbooks are actually quite cheap compared to other professions like commerce, law etc. Support those who helped us come this far...
2. If you want free information, try Google. I just did "concave-convex rule" and came up with loads of useful hits.
Otherwise the textbook that you used for university (anatomy and musculoskeletal/orthopaedic physiotherapy) will do. Amazon sells a range of these books.
Good luck