Try your local outfitters, construction supply store, or farmers supply store and look for black denim pants (jeans). I've seen them at Fleet 'n Farm (or Farm 'n Fleet, if you prefer) and Cabela's. It's rare to find them in the carpenter/cargo cut (with the extra pockets), but hey, take what you can get.
Or just buy the cheapest pair of khaki cargo pants you can buy and replace as needed. Or get one of your costumers to patch them for you.