Walden, Mitchell, Custom Hide, Filson and Coronado Leather all make some heavy duty leather bags. J Peterman offers the Counterfeit Postal Bag styled after the USPS all leather bags of the past. It's pretty nice but I have seen other takes on the postal bags that I like better. Check out ETSY for as there are usually a few artisan made leather postal style bag as well as other styles there. J Crew used to offer the Artisan Messenger bag, a oiled leather postal bag variation, which is heavier duty than anything you might expect from them. It has been discontinued but they show up on ebay often and that is where I bought a new one about a year ago.
You might also want to consider having a custom bag made by a US maker out of US leather to your specifications. Most of the heavy duty leather bags, including those from Saddleback, use 4-5oz. cowhide but you are certainly not limited to that with a custom. Coronado makes some bags out of 8-9oz Bison leather.