Reddit reviews Flints by Ronson - 5 Flints

Buy these.
https://www.amazon.com/Flints-by-Ronson-5/dp/B006MKDGNA

Not from this link--they'll probably be cheaper locally.

Pull the lighter from the shell. Underneath by the wadding, you'll see a brass screw. Remove screw, and you'll find a long spring on the end. At this point, the jammed nubbin of flint under the wheel can be removed by spinning the wheel in the opposite direction. Should just fall out.

Put a new flint in the tube, replace sping & screw assembly. Body back into shell. Resume practicing Zippo tricks. You could even fit a spare flint behind the first, but depending on the spring this can make the wheel harder to turn.

Speaking of which, the first few flicks of the flint may be difficult to strike. That's normal and the flint will "break in" fairly quickly.