I'm looking for a place for my loved one with dementia/ Alzheimer's that is locked. Any suggestions?

That kind of facility is called Memory Care. It is secure to prevent wandering.

Dedicated memory care facilities are still rare in my area but all nursing homes are secure and many assisted living facilities have a secure floor or wing for those with dementia.

In our area as with CWillie, memory care facilities are scarce and are also private pay, just like assisted living but even more expensive. Maybe your area is more fortunate. My Moms NH has a Memory care floor. But any resident of any floor can be equipped with a wander guard attached to walker, wheelchair or wrist which sets off an alarm if they try to get onto an elevator or out the door. Elevators will not move if the alarm is sounding and door will not open. Not sure if your LO is appropriate for NH care, but it might be something to consider/investigate.