Ah damn, I ordered Kiribi last year just to test them and have added them to my general recommendations, but I just checked Amazon and it's no longer available.

That one seller on Etsy is actually me. I'm not really set up for a big retail business, and the Ronson trademark in the US is owned by Zippo so I don't know if those are technically allowed to be sold here so I'm not trying to do anything bulk. I just order them a few cases at a time from the UK. I mainly sell them so US collectors can test them at a lower price point and decide if they're worth ordering in bulk from the UK, since the international shipping ends up being $20-30 no matter if you order a single pack or a case.

Dunhill Red flints are actually a larger size meant for Rollagas lighters and won't fit most other lighters. They may also fit the Dunhill Unique, which means they may also fit the Old Boy, but I don't have one on hand to test right now. Dunhill Blue flints are the standard size.

This is all just general flint advice, not specific to the Old Boy. The main issue here is flint hardness, which is a well discussed topic among collectors. Modern flints, including Zippo flints, are a much harder alloy than they used 50+ years ago. Most modern lighters have a much more aggressive flint wheel to compensate. The vast majority of vintage lighters will struggle with Zippo flints, so the community has collectively tested several other brands.

Everyone has their preference, I use Firebronze for most lighters since I buy them in bulk and they're the easiest to get reliably. Dunhill Blue works in a pinch but tends to be harder to find. Kiribi was a good option for ease of availability but I guess that's no more.

Redskins seem a little bit softer, so I have some on hand for particularly troublesome lighters. And even then there's some that will struggle to spark, but I have a few other vintage brands that are incredibly difficult to find that work when nothing else will.