I have changed my oil several times and never needed to change the drain plug or the washer O ring. Just inspect it for damage, smear a little oil on it and put it back in.
If you do need to replace if the head is damaged for instance then this from Amazon will do the job :-)

https://www.amazon.co.uk/Blue-Print...mp;psc=1&refRID=JVXMRBZ28NFF640B3TPE

Paul

2009 4/4 Sport in Red
2022 Harley Davidson FXLRS Low Rider S
1977 Triumph Stag