Some history on the "Cal. 30 US"
Problems with the .30-03 resulted in production again being halted in order to adopt a new cartridge. The .30-03 had a round-nose bullet eventually proven to be inferior to the sharp-pointed German-developed "spitzer" bullet. In October 1906, a vastly improved cartridge with a "spitzer" bullet and reformulated powder to reduce the .30-03's severe bore erosion was adopted as the "Cartridge, Ball, Caliber .30, Model of 1906," now widely known as the ".30-'06." Existing barrels were rechambered for the new cartridge and all subsequent M1903s were in .30-'06.
Prior to the 30-03 the predominent cartridge was the 30-40 Krag, a rimmed cartridge that has never been one of the most popular cartridges in the USA. The 30-03 was a bit different from the .30 US model '06 in that it had a longer neck. You can fire 30-'06 in a 30-'03 but not always the other way around.
As to where you can find a barrel in the .30 US caliber you would have to decide which of the military cartridges you wanted to use - the obscure 30-'03 or the common 30-'06. The barrel twist remained 1 in 10 for both cartridge / rifles and it is still the standard twist in 30-'06 today.