A half-circle window is not cut with a regular knife. It is mainly a special-order item.
You can always install temporary plexiglass and use different cutting methods as a short-term solution.
In Pembroke Pines, Florida, there is no factory that fabricates this type of window on demand, but you can find local suppliers that sell plexiglass to use temporarily while you wait for the permanent window to be ordered and fabricated.
https://sliding-glass-door-repair.com/pembroke-pines/window-repair
Also here is a local plexiglass source: http://pembrokepines.businesslistus.com/specific/Polymethyl-methacrylate-resins,-plexiglass.htm