A green coffin is made of biodegradable materials — unfinished, sustainably harvested woods; willow; sweet grass; wool felt; bamboo, or the like. Check in your area for woodworkers who build green coffins (to reduce transportation costs) or check on-line for kits, plans, or completed coffins. In central North Carolina, I recommend Aldergrove Woodworks and The Green Man Casket Co.
For other areas, check the list of product providers on the website of Green Burial Council.