I'm looking for data on Medicare Advantage Plans: eligibility, cost, coverage, etc. Where can I find it/
The plan finder on Medicare.gov has some of the information you seek. It is only a general overview, however. It is missing some of the specifics, such as costs for certain types of medical care. I have used it to help some people in the USA find appropriate coverage. When doing so, I did find some of the information to be inaccurate. I also found that sometimes wide ranges of costs were presented for some medical care/treatments/procedures, which was not helpful. For example, I once saw something like $0-$800 specified for a plan's medical imaging copay.
The following website has data on estimated costs, by region in the USA, based on approximate anticipated health status: http://www.medicarenewswatch.com/index.php/en/cost-comparisons
Here is the FAQ explaining the data presented on that website: http://www.medicarenewswatch.com/index.php/en/faq
I am currently unaware of any data source that contains comprehensive, accurate, and detailed Medicare Advantage Plan information. If someone is aware of such a data source, I encourage them to add another answer.