I am getting desperate ...

The sandpiles and seashells spawn at nighttime so you should spend the night at the lot until they do (or move there with your Sims)

It's been a long time since I've completed these collections. Aside from digging in the sand, I remember occasionally finding random shells near the shoreline, and also one of the Sulani festivals will sometimes have shells on the picnic tables. I might have also found some in the cave, but I could be wrong on this.

have toddler play in the sand in sulani they find lots of seashells :wink:

or beachcomb in sulani with any sim of choice

buried treasure id suggest fishing in sulani and/or diving for treasure by the dive buoy or exploring the cave in mua pel am

Throw a Kava Party. The rewards are sea shells and buried treasure.

While you're out fishing, throw out a few fish traps also. I've gotten all kinds of treasures from those. Nice fish too.