I have seen a newbie selling an edge adapter including a MX4 slot, but the price is just a joke.
That person is probably me, thanks (depending on the definition of newbie - I had my first CPC 464 in 1985

) Or maybe there some other source for these connectors I dont know about

Anyhow - my PCB gerbers are online, so people can just make it themselves.
I only charge money for my time of assembling it. I am not making any money
with the products themselves, hence everything I am selling is shared on OshPark.
Further, in my offers Ebay and Sellmyretro, free shipping is included in the price, which is 14 $ from the US to Europe.