any 6mm copper washer will do. it doesn't have to be OEM honda.
the honda ones are actually more fancy, i don't remember what exactly - maybe brass coated with zinc? but copper works fine there, and is easier and cheaper to get. also on the rocker cover, those bolts that have oil behind them need sealing washers so better just get a few of them.
the bolts holding down the rocker cover:
i'd replace the jis with allen heads. be careful with torque on all 6mm bolts that go into alu on these blocks. it's very easy to strip the threads, especially with a torque wrench. it's enough to carefully nip them tight by hand.