How do you get temporary guardianship of a parent with severere bipolar, who has become dangerous?

You should see a guardianship/elder law attorney as soon as possible. You can petition the court to have yourself or another concerned person appointed as guardian. If you are appointed guardian, you will be able to help make financial and healthcare decisions for your mother in law. Also, with regard to finding a long term care facility, you should speak with the social workers on staff at the mental hospital. They will be able to help you find dishcharge placements which will be safe and suitable. Currently, it appears as if she is in the hospital under an involuntary committment. The hospital should not release her while she is a danger of harm toward herself or others.