Do you need a person teaching you directly? Then you need a mentor (to guide you) or to go back to school.

If you just need someone to have done the legwork and established the exercises and presentation of material for you, there are a myriad of options: books, blogs, videos, open courses, paid courses, etc.

The trick I've found to mastering a field without having a teacher standing over me and guiding me (and even when I have a teacher) is to learn to construct my own courses and exercises. These are "synthetic". I look at multiple university courses on a topic and grab one or more books, and then work through them in an order that fits my way of learning. If they have a lot of exercises, I do them. If they don't or their exercises are poor (happens), I have learned (over the years) to construct my own exercises.

A major lesson from years of doing this is that there are no perfect teachers, courses, books, or other resources that will, on their own, teach me a topic to mastery. I have to assemble it myself to fit my way of learning and needs as a learner. That part requires introspection along with trial and error. I attempted courses and books that taught me nothing (or barely anything). After a bit of floundering with that, I took a step back and figured out what resources were effective and ineffective for me, what they had in common or were missing, and started assembling my own "courses".

That's a very useful skill to develop, and it transcends disciplines. Even if you do have an actual teacher with you this is still helpful (perhaps even essential) to be able to go beyond what they're teaching you and to cover over their weaknesses as a teacher (either general weaknesses or weaknesses with respect to you).