Contact the DDS office nearest to you (reach out to your school system for help with this if needed). They have caseworkers who help with this transition, and also with the process of legal guardianship after 18. There are dayhab programs, residential programs or support workers who DDS can pay for to help with all of this.

I work in elementary sped now, but a few years ago discussions about aging out/next steps was a requirement at every IEP meeting after age 14.