Industrial strength zip ties are available at Heating and Air-conditioner supply houses.
Marine grade stainless steal clamps are available at boating and marine supply houses and plumbing supply houses..
Various plastic clamps are available at landscaping, irrigation and swimming pool supply houses. Some of them will have marine grade stainless steel clamps as well.
The big box stores, Lowe's, HD, etc. Usually sell lower end items, even their "Pro- Grade" labeled stuff is low end "pro."
Swimming pool parts have to handle high pressures and concentrated chlorine, pool supply houses are a great source for other wise hard to find items, and prices are usually cheaper than similar things marketed specifically at aquarists and sold in specialty stores and pet shops..