The first time I visited the shop, all the lights were on but the owner was nowhere to be found. "He comes and goes a lot," the kiosk owner next door told me about the shopkeeper. "I think that he has a large family and that takes time away from time in the shop. Most of his customers arrange visits in advance."

After two weeks of trying, I was finally able to get hold of the owner of this shop by phone. He was dismissive, clearly agitated by the waves of Turkish journalists eager to visit Istanbul's only known purveyor of ISIS merchandise. Given the growing number of Turkish nationals volunteering to fight for the ISIS, as well as the Turkish government's recent classification of the group as a terrorist organization, his reticence was not surprising.

Journalists are lining up to interview the ISIS clothing salesman

Partly due to its far-flung location, Islami Giyim managed to sell ISIS clothing for months without raising the suspicions of local or foreign journalists. All that changed after ISIS's summer takeover of the Iraqi city of Mosul. Now journalists are lining up to interview the ISIS clothing salesman in order to demonstrate how entrenched the militant movement's foothold is in Turkey. The Turkish media stories generally cast the shop as an example of ISIS's growing reach beyond the borders of Iraq and Syria.

"I am not an ISIS member, nor have I ever been one," the owner of the shop said through a translator. Refusing to give his name, he expressed surprise at the surge in media attention that his shop has stirred up in the past few months. "I am responding to a market demand. This is Islamic clothing. What else can I say?"

The ISIS logo. Clothing targeting young men has come to be a central component of the group's branding.

One of the most popular items on sale at Islami Giyim is a black T-shirt with the slogan "There is no God but Allah" written in white letters. The logo is used widely by ISIS militants on flags and banners. When I asked the owner about it, he curtly said, "This is Islamic and that is why I carry it here." When asked about the ISIS connection to the logo, he quipped, "For some this brings to mind ISIS and jihad, for me I see the Prophet Muhammad."

The idea of ISIS T-shirts shouldn't actually be that surprising. For a terrorist organization hell-bent on creating a state based on a puritanical and bigoted form of Islam, ISIS has an incredibly savvy marketing and branding campaign. The group uses social media platforms, especially Twitter, to mold a specific media narrative and recruit funds, as well as fighters, from across the globe. Clothing targeting young men has come to be a central component of the group's branding. By most accounts, the production and design inspiration of ISIS clothing stems from East Asia.

The Indonesian company Zirah Moslem has emerged as the world's leading seller of ISIS merchandise. T-shirts are generally priced under $15 and, until recently, could be found on its Facebook page. Before Facebook removed the page for violating its terms of service, Zirah Moslem had more than 9,000 likes. The company still sells clothing on its website and likely acts as a wholesaler to smaller operations around the world, like Istanbul's Islami Giyim. While it might be easy to buy ISIS T-shirts and other articles of clothing online, to date Islami Giyim is the only brick-and-mortar establishment to receive any press.

The Zirah Moslem logo from its Twitter account, which also has the description 'All about islam with a T-shirt,Hoodie and jacket, Islamic style movement…'

The popularity of ISIS clothing—most notably T-shirts emblazoned with the group's initials flanked by AK-47s—demonstrates the next phase of ISIS's international branding campaign. Islami Giyim's Facebook page already has 6,400 likes and prominently displays a variety of ISIS-related clothing items. Without revealing exact sales figures, the store's owner said that business was good. So good, in fact, that he plans to open more locations throughout Istanbul in the coming months.

It's not hard to see why ISIS clothing has found an audience in Turkey. While Turkey has denied funding militants in Syria the way that Saudi Arabia and the Gulf monarchies have, the country has helped militants in other ways, such as allowing ISIS and other Sunni jihadi forces the use of territory to access Syria.

Staunchly opposed to Syrian President Bashar al-Assad, Turkey has largely kept open its 510-mile border with Syria. Foreign jihadis, easily spottable by their long beards, young physiques, and varied Arabic accents, have been using Istanbul as a main international air transit point to reach Syria. Towns in southern Turkey, such as Hatay, Reyhanli, and Gaziantep, have large jihadi populations. On a recent trip to Gaziantep, I watched as men speaking Iraqi Arabic bought thousands of dollars' worth of camping equipment from an outdoor store 15 kilometers from the border. After a cup of tea with the owner, with the gear loaded in large duffel bags, they departed in the direction of Syria.