Oversize, contractor-weight trash bags shouldn't be hard to find locally. I've even seen extra heavy duty ones on the shelf at big box hardware stores, though I don't recall if they were 4 mil.
Call around to your local janitorial supply companies. Make sure you tell them it's for scouts; you might get a freebie.
large heavy duty plastic bags can be found at Duluth Pack.used to WP their canvas portage packs they come in several sizes.these are very heavy plastic so don't expect one to fit in a pocket size kit.
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