Comeaud,
Brake parts can be a challenge. I had a hard time finding a new master cylinder for my 1966 GMC HM5500 because it combined the brakes and hydraulic clutch in the same housing, the size is larger than what was used in lighter trucks, and the original part numbers led to discontinued parts. I finally had to resort to finding a housing with the same bore and "stroke" measurements as were given for the original parts and the same bolt pattern based on available pictures. But it does work. You can also have your current unit rebuilt, with a sleeve if needed.
Since you have a big truck, it must have some sort of brake booster, which can further complicate your search for parts. Also, your truck is newer, so it may have a dual system that separates front and rear brake lines (this is a good thing). Mine is a single line system where the master cylinder line goes to a hydrovac booster located under the driver's side step.
Good luck! You are not alone. Others on this site probably have first hand experience with your year and model and can provide more advice about parts availability and dealer locations.
