Plugs are very specific , extra long thread , not a lot of options, and not cheap
If your running rich for cyl temp safety they will last less time, also may need to reduce the gap if your pushing higher psi numbers than stock or using E85
I did see the oem for £22 each, free shipping. Ebay I'm fairly sure. Motopartz or something similar. Definitely the right plug.
Be aware that there's a lot of fake stuff on the Internet. Certain countries don't appear to have any control over this, China and others are you listening? If it's advertised at half the normal price I would be suspicious unless the seller can guarantee it's genuine - and you can trust them.