Hi Lily, I join WDx in welcoming you too!
Are you in the US? If so, you need to start within your state, start

your county level. Our medical insurance does not have any residential benefits at all. We are in NJ and our D16 went thru a residential program recently. What we have found was some were better than others, but there were good options and it didn't cost our family a dime! I am not sure what services will be available since your D is over 18, but there must be something. A call to the county mental health services office may be a good place to start. Another suggestion would be to contact your local NAMI office.