Looking for my niece

My beautiful niece was born in July of 1991 in Joplin Missouri at either Oak Hill or Freeman Hospital. Her birthname was Amber Gail. Mothers name: Tiffany. Amber was given up for adoption (July of 1992) the day before or the day after her first birthday. (I was 16 at the time so some of my dates are ever so slightly fuzzy) I have no idea what method of adoption my sister used whether it was thru an agency or private? If someone could help me I would really appreciate it! I just want her to know that she is and always was wanted and loved by all of us!