[Roadsters] question on correct size mounting acorn nuts for roadster convertible top

[Mtbdavid2 at aol.com]

Hi,
  I cannot seem to find the two acorn nuts that held the convertible  top 
vertical arms that attach to the brackets behind both doors.  This  is for my 
1968 roadster. Does anyone know the correct size nut to use on the  
brackets which are mounted behind the two doors?  I assume they are metric  but I 
cannot find a nut from local hardware store that will fit.  Do you  know if 
any of the roadster vendors sell these two nuts?
 
Thanks,
David