I would contact a music department at a university, or composition department at a conservatory or school of music. Often grad students will take on a student composer, and sometimes a professor will too, depending on various factors. You could also contact a conservatory prep. So much is done online now, which broadens the possibilities.
As a side note, many PhD composers are adjuncts and would probably welcome more students to teach outside of the classroom. The above mentioned contacts should help connect to them as well.