Tubby,
Bit suprised that an ATPL is asking this, but here goes.
Airworthiness Directives can only be issued by an Aviation Authority, and are issued by:
a) the Airworthiness Authority of the State of Design (e.g. EASA for Airbus airframes, EASA for Snecma engines, FAA for CF6's, Transport Canada for DHC 8 airframes and PT6's etc)
b) the Airworthiness Authority of the State of Registration (e.g. FAA for 'N' reg, EASA for EU reg, Transport Canada for CF and C reg)
Service Bulletins are issued by the Manufacturers of either the main assembly (e.g. Airbus for airframe), or of installed equipment (e.g. Rolls-Royce for engines, Honeywell for avionics, etc)
If an Authority (e.g. FAA, EASA) consider an issue to be of potential risk to the safety of the aircraft/crew/pax, then that Authority may raise an AD. The AD may incorporate a Manufacturers SB, but this is not always the case.
AD's may be reviewed for free from most Authority websites (e.g.
www.faa.gov for the FAA AD's,
http//.ad.easa.europa.eu for EASA AD's.
Service Bulletins are the Manufacturers Intellectual Property, and you generally have to pay for this to access the information.
If you are in an airline enviornment, go see your Technical Documentation department or Quality Control department and ask to see the relevant SB's you are thinking of. After all, your employer has paid for them.
Hope this helps.
camlobe