Boom Town Hobbies has then listed on their site: [link=http://www.boomtownhobbies.com/BoomTownHobbies_O_ring_3pack_for_boom_kits_p/bth020.htm]Link[/link]. I recently bought some replacement fins from then, and it too came with the O-ring [link=http://www.boomtownhobbies.com/BoomTownHobbies_M3_replacement_fin_set_p/bth003.htm]Link[/link]

Like the others said too, take them to a hardware store they might have them if you do not wish to order.