We (the West) have been arming all kinds of bloodthirsty maniacs in the middle east and north Africa as of late, so I wouldn't put it past us to have armed ISIS as well. Having said that I was under the impression that a lot of the modern arms that ISIS is currently using were seized from the Iraqi army (provided by the US) when they ran away. However I do imagine their is a good chance that at least some of the arms we have been flooding the area with recently will have found their way to ISIS as well.