The official Honda set come with a protective sheath, and a gland, washer and coil screw cap. But the HT lead itself is standard, stranded copper core, which can be purchased quite cheaply
The coil end of the lead is a push fit onto a spike deep inside the coil housing. As such, it could easily pull off. The gland is glued onto the HT lead so that the coil screw cap keeps the HT lead pressed onto the spike. The washer is a plastic type, which allows the cap to be screwed on, pushing against the gland without twisting the cable or offering too much resistance.
The sheath which covers most of the length of the HT lead is glued at the plug cap end. The HT lead is screwed into the plug cap -about one inch.
If you want to use standard copper core HT lead, You can salvage most of the parts from the old HT lead, except the gland. When I did an emergency repair on mine, I did just that, and fashioned my own gland by tightly wrapping chord around the HT lead at the appropriate spot, pulling it tight enough to form a small 'waist' in the HT lead, and keeping it tight until the glue set. I daresay that a cable tie may work, but never tried that.
Any of the longer OEM plug leads will do. You can chop off the surplus length at the plug cap end, and trim the sheath bike to the desired length. Since it is glued at that end the sheath will become free to move.
The official plug is NGK manufactured, but NGK do not supply one that fits. They have a straight cap that is ling enough, but the lead emerging at the top would be a problem.