I missed the bit in the original post that said that the OEM had the parts available but that they were too expensive

Machine tool parts are always expensive – industrial machines particularly so. But the manufacturer can still supply the parts you require for your 40ish year old machine & they should go straight in, be made of the originally specified materials & to original tolerances & accuracies. The thread will most like be thread milled or thread rolled & may have been ground & the nut threaded with a special tap that initially roughs the thread undersize then finishes it (think of two taps joined together axially, with the first being a bit undersize & the thread being continuous).
As you say, there are cheaper options. WMH & Raine sell rolled acme screw stock by the metre in various sizes, leads & both hands. Bronze nut blanks to suit also.
Depending on how worn your screw is you could re-cut the thread to remove the wear & make a new nut to suit the re-cut thread – minimal outlay with that route (you could do it on your machine by locking the cross-slide & using the compound suitably positioned) & one used by a machine tool refurbishing company I was seconded to during my apprentice days.
Regards,
Nigel B.