Should I use an agency or have an individual help my mother-in-law, who is 91 and has dementia?

In-home care should be able to fill the gap in care for your mother-in-law. If you have choices, chat with them all. You want to be sure you have a good fit. Agencies can be good because background checks should have been done (ask to be sure), as well as the fact that they have many caregivers so if one doesn't work out, you can ask for someone different.
Individuals can be wonderful if you find the right person, but you are on your own with background checks and other decisions about whether they will make a good caregiver and rules about employment.
Good luck with finding a good caregiver. That should ease your mind.
Carol

I got my mom a round-the-clock caregiver and she has been wonderful. Mother's problems are more physical than mental. Since then, Mom is improving. The caregiver is worth every penny!