that sell plastic furniture and products but no info on raw materials.
Find a manufacturer who makes what you want (Google for 'HDPE plastic lumber') and once you locate one or two ask THEM who sells it within your area. This is far easier than going from the other direction. Sales people are usually eager to tell you where to buy their products and I've even known them go arrange free samples if I acted nice - I've scored free 'Lyptus' lumber, loads of hardware, paint, finishes, glue, tools, and other goodies after talking to sales critters.