An old style bicycle shop should also be able to supply a cotter pin with no flat at all which you then file your own flat on to suit your particular application. What you are looking for is absolutely bog standard for old style bicycles. There is nothing fancy or different about your cotter pin because it is for a motorcycle.
Unfortunately most modern cotter pins are machine made and have the same size flat on them. Hence the need for an old style bicycle shop where the staff know old bicycles. They should also be able to show you how to correctly fit a cotter pin. Like everything else about vintage machinery there is a certain degree of knowledge and skill required if you want to get it right.
As Carden says, they are exceedingly easy to make if you have access to a lathe.
Cheers,
