android - how to access files, and where to put them?

i want to be able to manually put files in some folder that my app can read but i do not know where. the idea is to have many levels in that folder and from the app to select what to load. i would like that to be as extension, to be able to put more files there manually or download directly to that folder from the net or market, the thing i do not want is that those files to be included in the build. how would i solve this problem? any help appreciated! thanks!