For those who follow, here is what I've done.
As I indicated at the start, I found a collection of camera shutter transitions. I then found a camera shutter sound clip at SoundBible. However, I then found I had to do some tweaking to get everything lined up.
The audio clip was 2 seconds but only had sound for 1 second. I had to change the slide transition time from 2 seconds to 1 second.
I had to add a 0:00.500 second offset in the audio clip to align with the transition with the sound.
I'm only using this custom transition for selected slides otherwise it would be a pain having to make all these adjustments for each slide.