Niah's is in the Waves Event park, which is between Lazy Dayz and Tony's bar, not quite the middle of the beach. It's the last "hut" on the left as you're walking in from the beach. If you go, make sure to take shoes to wear once you leave the beach

Earlier in the day is best, especially during the off season, he'll have more available then. He tends to run out later in the afternoon.