Hi Xenia,

I see you've already come across a couple of Alzheimer's disease data-sets, but you might also be interested in the longitudinal data from OASIS:
  http://www.oasis-brains.org/

The Dominantly Inherited Alzheimer Network will also make longitudinal MRI data available, though I believe there is a delay between data being available to DIAN investigators and it being more widely available, and I'm not sure how many subjects with longitudinal data are available yet.
  http://www.dian-info.org/resourcedb/

Best wishes,
Ged