SPARTA is an organization for transgender service members. They provide guidance and support for trans service members https://spartapride.org/

Lambda Legal is the law firm representing transgender service members in Talbott v Trump. They are a civil rights law group that focuses on LGBT people https://lambdalegal.org/

The ACLU has been fighting for the civil rights of all Americans for Decades. They’re also involved and coordinating and recording injustices. They’re not trans-specific however they are always a good group https://www.aclu.org/