Where do you find the conch shell you blow in?

They're on the sand in at least 3 places in Ohan'ali Town. Its initially just one in each spot, but if they're not picked up every day, you'll get several shells in a pile, like I saw in my game a couple hours ago:

https://i.imgur.com/oZDklfw.png

There's one pile towards the Kealoha house, then one closer to the fishing net and the last one is across the water from the beach lot.