My understanding is anyone who can issue a FROL can issue an ARCOP.
Local flying school should be able to sort you out (or the organisation requiring you to get an ARCOP should be able to point you to who they normally use).
Most places end up using Flight Radio for Pilots from Aviation Theory Centre as a guide. Can usually be knocked off in an afternoon. Cost depends on who you do it with (a carton, or their hourly rate) plus $50 to CASA.
Syllabus is here
www.casa.gov.au/fcl/download/arocp.pdf
Form for CASA is here
www.casa.gov.au/fcl/form638.pdf