You’ll need to pickup 608 type I certification to legally buy most refrigerants. It’s inexpensive, the exam is open book, and takes an afternoon to complete.
The “textbook” used is actually a useful reference if you’re just starting out. The material familiarizes you with common terminology, regulations technicians must follow, and the procedural basics for typical jobs, but the emphasis overall is how to handle refrigerants safely and avoid venting them into the atmosphere.