Honda Parts Direct, I think is run and approved by Honda, they used to sell boxed Service kits, 8 plugs, 3 filters Oil/Air/Air conditioning for a bargain price, all OEM Honda packaged parts.
They seem to have dropped the kits but a set of 8 spark-plugs is around £51, see image.
Probably a safe bet to buy the spark-plugs from the source, remember there are many fake spark-plugs purporting to be genuine NGK, those fakes are hard to spot from genuine plugs, I'd feel happier with parts in a Honda box from Honda than a general retailer