If I need to go to the hospital on an emergency basis, who can I get to watch my husband who has dementia? My children cannot help.

If family cannot step up to help, you will need to hire private caregivers. Call a reputable agency who screens their applicants. Ask them about emergency in-home care when and if it is needed. They can usually get someone to your home on the same day. We paid $19./hr. where we lived.
Also, if you anticipate needing in-home care, make sure to secure valuables such as jewelry, cash, and important papers in a safe place. The manager of the agency we used suggested this to us so that there are no "misunderstandings."
You could also contact your senior center, office on aging, local churches, or other charitable organizations to see if they have any emergency volunteer help. Do you have a local Alzheimers association? I understand some states have respite funds for Alz. patients.
good luck