Reddit mentions of Pure Paws H20 Shampoo-1/2 Gallon

Owning a grooming a newfoundland, the most important tool is a GOOD blower/dryer. They are not cheap but you can justify it if you can save money by doing groomings yourself. They are very powerful blowers but don't have a heating element like a dryer for people, just the heat from cooling the motor.(K9 Dryers are the goto for pros, fully repairable for life) You should be able to blow most of the dirt off without needing to bathe that regularly, in fact blowing the dirt out should always be the first step before you get them wet or you just make mud on the coat when bathing. If you still feel like you need to bathe that frequently I would look to some of the higher end shampoos used by show dog owners. They are typically bathed far more often than pets and won't dry out the skin as much as a grocery store shampoos. I really like the pure paws H2O line(extra moisturizing), and they have 1/2 Gal bottles available on Amazon. Good luck with that dusty dog, I know how it goes...