There are dozens available for normal subjects and different diseases so it would probably be easiest just to google and find ones with a license appropriate to your needs.

The simplest to use and most open ones are:

• OpenNeuro: https://openneuro.org/
• Human Connectome Project: https://db.humanconnectome.org/

OpenNeuro contains many datasets collected by different people in different populations and experiments. On the Human Connectome Project database you can find datasets collected by that project, most notably their large healthy young adult data release.