Is this possible? If so any advice on where to buy?
I was under the impression that Apple had locked the gates of the walled garden and it’s only possible to use their certificates for signing a .pkg installer. I may have misunderstood this restriction is only for apps distributed via the AppStore.
The cheapest certificate for signing Windows installers doesn’t say it can sign macOS, so I guess I’d be looking at one of the more expensive options if this is possible, in which case it’s about is the cost of a cheap certificate + Apple Developer membership less than the single cert