Two points I would raise for consideration -
(a) there are copyright issues involved in providing/accepting/using a copy of a copyright document
(b) regardless of where/how you might source one, please do be sure that you have the correct one, ie S/N and configuration applicability.
On another tack, the airframe OEM's POH usually will have naught to show for other OEM STCs and you will need to source any relevant information relating to STCs from the STC holder.