You may already have had a response - but here's another 2 cents:
I've lived in St Augustine since a kid and recently gotten more into the
shelling. But I can't say there's a good place for good shelling around St
Augie. A little north - between Vilano and Ponte Vedra I can find whelks when
tides are right - Off of Anastasia I can find sand dollars of other small
things of interest - but no rare finds. Further south towards Flagler may be
the same scenario. Most of us tend to go to Sanibel, Keys, .... other places -
but you can always have a good hunt wherever you are.
Good luck