Briefs, in my experience cover a vast gamut of scenarios, from self-briefing to a fully blown hangar full of people. As such, finding a template is difficult, so indeed is finding the ideal brief for a given sortie.
Preparing the brief (briefing material) is key.
Remember the 5 (or 6) Ps - Proper Planning Prevents etc.
If enough fore thought and preparation goes into the sortie content, then the content of the brief itself can be generated relatively easily - making sure all the relevant people attend the brief and understand what is going on (when and why) is the hard part. That requires a good briefer. A good briefer is (and sounds) knowledgeable and exudes an aura of confidence.