If your goal is the Ph.D., getting job as an Research Assistant for a couples years is your best bet. Having a masters generally doesnt increase competitivness unless you have to offset a low undegrad GPA. If your GPA is solid and you are confident you want the Ph.D, dont waste the time or tuition money on a masters program. You will get alot more research experience and possible publications working as a research assistant for a couple years. This is what increases competitivness.
If you still deciede to do a masters, simply look for a faculty member who has some kind of interest in AD. You didnt say what about AD you're interested in (ie., neuropsych and diagnositic issues/assessment, caregiving, quality of life, genetic risk factors?), so I cant direct you to anyone in particular.