This is a planter I made from scrap pieces of composite decking (TAMKO Evergrain - to be specific).
Composite decking makes great planters because it lasts forever. The only down-side is the cost - it is quite expensive.
If anyone in your neighborhood builds a new deck, stop over and tell them you would like their scraps when they are done. After they look at you like you're crazy they will most likely be happy to get rid of it.