Tire tubes are a good source of rubber - a flexible membrane for fashioning a pipe-collar like boot around a trunk that passes through your tree house roof. Please carefully cut the tire with a sharp knife, and each tube will yield two rings of rubber with a natural curve to them that will prove useful when wrapping it around a tree trunk.

These are used truck tire inner tubes. Some of them have patches, markings, and are different shades of gray & black. They are generally dusty/dirty as pictured above.