I've seen (on home improvement shows) shelves that hang with hidden
brackets (ex: Mount a 2x4 onto a wall and slip the shelf over it and
secure from top (obviously, the shelf is about 4" thick). I've also
seen (again, on TV) shelves that look like you screw supports into a
wall (short tubes?) and slide the shelf over them and secure.

Where can I find something like this or do I need to build my own?