You can get custom keycaps made up by WASDkeyboards.
I would, of course, recommend switching to unicomp (Model M) as an alternative. They sell keyboards for apple with those legends.
For example: http://pckeyboard.com/page/UKBD/UNIZPHA
I'd like to have a tenkeyless keyboard and they don't seem to offer one at the moment.
I tried WASDkeyboards but there are only a few pictures I can chose for legends and there are none for volume control etc.
the media keys are nice, thank you, but I still need some more for mac