Vintage cars require mostly smaller sizes. Usually these are available in coarser sizes only and are often too long, at least for Repco and Bunnings.
A good source of stainless steel in smaller diameters and lengths is from an aircraft supply shop located in most of the aerodromes where smaller aircraft are serviced, such as Archerfield, Moorabbin, Bankstown and Parafield.
Alternatively they can be ordered from Aircraft Spruce and Speciality from USA, who ship next day after receipt of order. Look them up on Google.
TIP: Americans call them 'cotter pins'.
Split Pin Fitting
As pins are fitted through castellated nuts, which hide to drilled hole in the bolt until almost fully tightened. This can be a problem if the bolt is in a difficult position, as they always seem to be.
The tip here is to file a line across the threaded end of the bolt, which corresponds to the line of the pin hole and in more difficult cases to also mark the bolt with a coloured mark on the tip of the bolt close to the hole position.
Sometimes the pin hole does not expose enough in the groove of the castle nut to allow the split pin to enter. This can happen because it is a new installation or a thicker washer has been used in the assembly or a shim has been fitted to get a correct clearance elsewhere.
Rather than fitting a smaller diameter split pin, the tip is to tighten the nut fully and mark which slot just aligns with the pin hole either from seeing the pin hole partly exposed or by the filed mark you made previously on the end of the bolt.
Remove the nut and file down that groove for the pin a little more.
For more severe cases the nut may be faced off in a lathe or simply filed off a bit on the clamping face.