The iJoy 26650 is more suitable for higher wattages so for 15 watts there exist much better available choices. Even though the HohmGrown 26650 suggested by
@fq06 isn't a poor performer, its performance still isn't exactly great, either.
The Vapcell purple INR26650 5900mAh 20A will give you noticeably more vaping time.
http://www.vapcelltech.com/index.php/Show/index/cid/37/id/37.html
You can get it from one of battery vendors that are on Mooch's list of trusted battery vendors:
https://www.18650batterystore.com/26650-battery-p/vapcell-26650-5900mah.htm
Sooner or later EVERY battery vendor is going to sell a fake, as none of the battery vendors have enough resources to be able to verify every single cell, and, what makes a trusted battery vendor trusted is what they'll do once they find out they've been selling fakes.
(If your plan is to use this Vapcell purple INR26650 5900mAh 20A battery in the Aegis mod by Geek Vape, then to make it fit inside probably you will need to replace the battery wrap with a regular thickness one... which doesn't take a whole lot of effort to do.)