aftermarket plug caps ?

[Carter]

Hi, traced the poor running to grubby carbs, all cleaned and set back to factory settings but found a duff plug cap then. Had a fright when I saw the yamaha prices so wondered if anyone knows of an aftermarket 10 ohm cap that will do the job. Failing that it's the breakers.
NGK did do a cap which probably would've been the right shape and size but looks to be discontinued so any ideas would be helpful.
Thanks
Nick.