Weapons, vehicles, employee salaries, propaganda videos, international travel - all cost money. IS's methods of financing are difficult to shut down. Terrorists use their control over a territory the size of the UK to develop revenue channels. The oil fields captured in Syria and Iraq fund the group through a black market oil trade flourishing along the porous borders of IS-controlled land. It is believed some traders have even sold oil from terrorists back to Bashar al-Assad’s regime. Because terrorists control an expansive territory, they can levy taxes on the people living there. Some of these taxes are more like extortion. The group is heavily diversified: if one funding source is shut down, they turn to others to generate revenue. A steady flow of money to IS from rich individuals in the Gulf continues, with Qataris the biggest suppliers. See http://www.nbcnews.com/storyline/isis-terror/whos-funding-isis-wealthy-gulf-angel-investors-officials-say-n208006