I made some out of thin brass wire that works perfectly.
I guess thin stainless safety wire would also work.
I have seen the very small cotter pins at Tractor Supply and Lowe’s.
Edit: I bought these at Lowe’s today = 3 for 58 cents.
mcmaster.com has cotter pins down to 3/64" (0.047"). But even those are too large for most old outboard carb linkages. They used wire for that application. Rip open an old coil and salvage the primary winding for a big supply of nice enamel coated copper wire.