Has anybody came across examples of Square, Acme, or Trapezoidal threads being milled?

I've got an idea for something I'd like to produce, and although I could use some standard metric threaded rod, I'd quite like to use something quicker/squarer. And that is where the problem starts.

Finding square threaded rod is akin to finding a blood donating stone. Acme and Trap on the other hand are available from a couple different suppliers for reasonable prices, which leaves me the problem of machining the corresponding female thread.
I had been considering 10 or 12mm, but even going up to 14mm to get a bigger minor diameter/hole size has yielded no results for thread mills. Given the width of the thread form, I fully expect to need to do the thread in multiple passes to get the required thread width.

Tapping in the lathe as a second op is looking like the only option, but given the length of taps involved, it's something I'd rather avoid, especially since one part will be awkward to quickly mount as it'll be rectangular.

I don't mind spending £2-300 on suitable tooling, but without going to full custom tooling, I'm drawing a complete blank. Anybody got any suggestions?