I generally use these websites for 18650s, both are good:
All Lithium ion batteries 18650, 21700 and 26650 sizes. Flat Top, Button Top, Tab's. Reliable source for all your 18650 battery needs. UK stock, Fast Shipping!
www.nubattery.co.uk
Buy UK Best 18650 3.7V Battery 26650 21700 16650 14500 18350 18500 16340 10440 Lithium Rechargebale Batteries & Chargers. Free shipping for all orders over £45. Next Day delivery available.
www.ecoluxshopdirect.co.uk
Stick to brands like Panasonic, Sony, Samsung and Sanyo for the cells. The Feneix cells are most likely are rebranded cells from one of these brands anyway. To be used in a light you're not worried about high drain (like you would be for either a vape or powertool), so go for the highest capacity you can (something like 3500mAh).
Be a bit careful about the size of the cell - protected 18650s are slightly larger (obviously) than none protected cells and some wont fit properly in Fenix lights. I seem to remember the protected NCR18650B cells (Panasonic) fit, but don't quote me on it!
Be careful with trying to fix 18650s unless you know what you're doing - there is a high probability of fucking up and them shorting, which is generally bad.
Hope that helps!
B