For starters, here's a map:
(or google "tree spirits of st simons island")
The first two are in the back yard of an abandoned house on Demere Road across from Baisden Lane. They're hard to find, because they're so old and reclaimed. The third one is right beside Murphy's Tavern, across from the Dairy Queen. There's one in Red Fern shopping center off of Frederica Road, in front of a restaurant. Another is in front of Magnolia Manor, further out Frederica, in the parking lot.
Back down near the shopping village at the pier, there's a cluster in what I think used to be a park, but now is a new development of houses. It's a couple of blocks back up King's Way. The trickiest of the ones I actually found is way up Frederica, where it turns in to Point Road (or West Point Road, I can't remember), at the end of a side street called Middleton Place. It's right up by somebody's fancy home, you have to go up their driveway to see it. There wasn't anyone around to ask, so I ran up the driveway, took the photo, and ran away ;-)
On the map you'll see one further south, off of Sea Palms Drive. This one is apparently back in the woods by a creek. I didn't go tromping back there. I've heard it's easiest to get to by kayak.