I know there was some discussion about these a while ago but can't find the original thread. Those who went to the Mega will probably have seen some cars driving around with quite creative Geocaching signs on the doors. One company that does these is Vistaprint.
I noticed today that the Catchoftheday site is offering $75 of credit at Vistaprint for $15, so if you are interested in getting some magnets (mine need replacing and one blew off on the way back from Albury) this might be worth a look.
I am not connected in any way with either of these sites, apart from getting the Catchoftheday emails