In terms of getting your hands on them, there's a decent selection of diapers available via amazon. They come in standard amazon boxes so they're pretty discrete. Most other vendors should offer some form of discrete shipping, I know at least abuniverse does.
In terms of keeping them hidden, most diapers should be fine. They do expand when you wet, so be mindful of that, but in most cases you shouldn't have to worry about your diapers being noticeably thick unless you've gone out and sought ones for that reason