HI, I am posting a link to the Hawaiian Gardening forum for you. I think you will more than likely find some more local resources from some gardeners that happen to be a little closer to home for you. One thing to note on the moss covered river rocks, you could always clean them up with a light bleach and water solution if you don't like the green growth. I have moss growing on many of my small to mid size rocks in the bed and it does lend a nice contrast in texture here and there.

I had a friend who put in a rock garden and every weekend she would drive down to the river and load up the trunk of her little Corolla with the rocks that she found there. If it is algae on the rocks I don't believe it will survive long without the water anyway. Her beds look great.

Here is a link that might be useful: Hawaiian Gardening