Seeking source for wheel joining studs

[Howard]

Ganmain Tony had several made up by a machine shop
I cannot remember what the costs were but he turned them & then fitted them too, for what I thought was a reasonable price... Try a local machine shop.
HH

[Grant Bowker]

I may end up having to get them custom made but it seems that if there's an off the shelf solution that can be either used "as-is" or modified to suit that it should be simpler and thus cheaper than a fully custom solution. One possibility I was thinking of was to take a standard bolt and turn down the head to either the mushroom shape used on the 20" rims or a suitable shape for the countersunk style on the 16" wheels.