For several years in the Syrian War, a CIA program saw US and Saudi-acquired weapons smuggling into Syria, nominally for the "moderate rebels." The program was ultimately scrapped this year, and had been criticized because the "moderates" never amounted to much.
But studies from Conflict Armament Research set about to find out what happened to these arms shipments. In many cases, the weapons wound up in the hands of the ISIS movement, and served as a source for some of their most advanced armament.
The biggest source of weapons ISIS had was looting the Iraqi and Syrian militaries, though in the case of Iraq this was basically the same result, as the US provided materially all the weapons for Iraqi forces as well.