I like the Staedtler Mars Lumograph leads. They come in all different hardnesses and go with standalone lead holders. They’re just graphite so not sure if it’s exactly what you’re looking for compared to the verithins. Then you have to get one of those weird little spinaround sharpeners, but there’s no wood wasted, and it doesn’t fill up as often.
You can buy a pack for $2-$4 and it has 2 or 4 full length leads.