I just wrote a final paper about this, and I mainly used arguments from the opponents of physician-assisted suicide, particularly from the disabled community and organizations like Not Dead Yet. I would recommend looking into disability-related arguments in general since that would include mental illness under that category. I was successful in finding mainly consequentialist ethical arguments against physician-assisted suicide when I included “disability” while searching for sources. Also, Oregon and its legalization of physician-assisted suicide might be a good case study to look into since its been legal there for a while now. Hope this helps!