Originally, the equipment came from overrunning Syrian bases and then capturing all the heavy armour, artillery, etc. on the Syrian bases. There are a number of instances in which they've overrun those bases. When they advanced into Iraq in June, they captured a great deal of territory, including overrunning a number of Iraqi bases. That helped them to get a lot more in the way of heavy armour during that period and they've overrun some bases since then.
But this is an area where they're in trouble because they've failed to overrun any bases for the past few months. They've made some serious attempts, including one back in December to overrun Assad's air bases, but ended up getting rebuffed.
One thing which I think is incredibly important to watch is when they make advances against bases, because the reason they're doing so is to try to get the equipment that's stashed on the base. That's the only way right now that they're able to get that specific weaponry. Now, there are other weapons they can get from other places, things like surface-to-air missiles. Unfortunately, there's a much bigger market for those, but tanks and Humvees are pretty hard to come by, and there are only a limited number of places where they can get them.