Greeshma, I don't think you can obtain these datasets for free online. You need to contact the neuroimaging centers to see if they can share some data with you. For instance, the neuroimaging center I worked at has this policy of storing deidentified brain data of participants for future sharing with researchers from other neuroimaging centers. Do do so, you need to contact the director of the center and complete relevant paper works. You can contact the Duke-NUS graduate medical school in Singapore. They have tons of data.

Ok, Thank you Jimmy.

Greeshma,

as for fMRI datasets, you can find some here:

https://openfmri.org/

http://umcd.humanconnectomeproject.org

Best regards,

Michał

You can find a large archive of physiological data here:

https://www.physionet.org/

Access to patient data is not such a simple thing. You must organize these things and have the appropriate permissions.