Their here....
Get your Miata Antenna Delete Plugs for less than the gas to travel to the Junk Yard...
$6.00 plus shipping
http://www.roadstertech.net/Our-Products/Antenna-Plugs/
Antenna Delete Plug on an NA 1990 1997
Antenna Delete Plug on an NB 1998 - 2005
if you have the original Antenna Seal. All the Better!
This Plug fits nicely inside of the seal and makes a nice OEM like finished Plug.