Two ways I can think of for you, Porkie.
Use a standard door frame (Imperial), for a standard 30" x 78" door.
The finished with is 845mm (33 and 1/4"). Easy enough to rip approx 10mm off each side of the frame to make it fit your opening (32.5").
But, you would also have to remove the head of the frame, cut the jambs (uprights) by 7" (to allow a 76" door to be fitted), then re-fix the head of the frame.
A standard 30" x 78" door would fit widthways, but would have to be shortened to 71" to fit the shortened frame.
It's the sort of thing I could do myself easily. If you're paying for someone's time, it may not be cost effective.
It may well be just as well to have a frame made to your exact dimensions.
I would take the advice of whoever is doing the job as to which he thinks is the cheaper option.