Science Buddies has some great resources on advanced projects--see the advanced project guide on this page: http://www.sciencebuddies.org/science-f ... l?From=Tab
Since I am not in that particular field of research, it is somewhat difficult for me to tell what may be going on. University professors are busy, and they receive many requests like yours. Taking on a high school student is typically a lot of work, but they get very little credit for doing so.
You say you have sent out 40 e-mails...have all 40 been to professors studying Alzheimer's? If you are sending them to other professors, they are unlikely to have the tools and expertise necessary for you to do the research you want to do. Also, have you taken the time to read their research, to see what it is that they are interested in? Generally, professors respond much more positively if they know that you are interested in the same things that interest them.