What platform style are you looking for? DPMS or Armalite
There is also another that uses H&K G3 magazines,so you really need to know what you want.
You will also need to match all of the other components to whatever lower you get,the AR10 platform isn't like the AR15 platform where everything is compatable with different brands/components.
Unless you're just wanting something different,it's easier just to buy a complete rifle in this platform depending on the caliber and configuration that you are wanting.
Texas State Rifle Association
North American Hunting Club