You're right, here in EU Honda is not selling them anymore.Suprised you found one. Those things difficult to find and usually involve buying the whole used needle assembly.
Was looking for a stock one, but if there is none, it would be the next step.If you have an old one, take it to an auto parts store or a motorcycle shop. They'll be able to match it up. Fiche lists the size as 2.8mm but o-rings usually are listed by internal and external sizes.
You are right, rubber NOS are not necessarly the best thing to use. BUT it's better, much better, than nothing ;-)Don't use NOS rubber parts, they have a limited shelf life. Looks like a honda generator part. $4.90 shipped on amazon. https://www.amazon.com/Honda-16175-260-004-Original-Equipment-Manufacturer/dp/B00NNWV4E8
You are right, rubber NOS are not necessarly the best thing to use. BUT it's better, much better, than nothing ;-)
And if you follow your link, you'll be able to read this:
We don't know when or if this item will be back in stock.