That's a 28" door, which is unusually small. You sometimes see them on Victorian cottages, as people used to be smaller. External doors are more often 32", but 30" and 36" are not unusual. Measure the brick-to-brick dimensions as well, because for best results you should fit a new frame as well.
I can't see one stocked by a big supplier, but a door specialist might have, or order, one.
I think you ought to start by asking around for a well-recommended and experienced local carpenter. She should know the local door specialists, by price and by quality.
Unless you choose to go to a boutique supplier, even a custom-made door need not cost thousands. There are colleges turning out joiners who will often have made a door from scratch during their training, the technology is not new or difficult. They can do it with nothing but a saw, a plane, a setsquare and some chisels, though it will be quicker with power tools. I had one made as a one-off about 18 months ago, the joiner told me he made it after work, in two evenings. He actually brought it as components he'd already prepared, and assembled in on my drive after measuring the frame and adjusting the fit. Assembly and fitting was finished by lunchtime.
There is a joinery works near me, they mostly do restoration work for the well-heeled owners of listed or period properties, but they whizzed up some wooden notice boards as a donation to a local charity, on a quiet day.