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.

Lowe's contractor bags 32-in x 50-in 3 mil Box of 50 $24.98

This site offers 2 mil 43x47 56 gallon orange bags for 59 cents apiece in boxes of 100. (http://www.interplas.com/43x47-56-gallon-hd-orange-trash-bags-p-cl-sho-4347h