I can't say if your particular door-size request is a custom one or not. I have a few 'places' you can check out where you can contact someone to ask your question. At worst, if they don't make the door, maybe they can make a custom one for you. It would be cheaper to have a Local Door Pro custom-build the door for you. That way, you don't have to pay for shipping (assuming the door is being made out of your area.)
Again, at worst, see if these folks will sell you plans or draw some up for your. (This is assuming a Local Pro can't do the job w/o plans.)
Beings your asking for a pre-hung door, be sure of the rough-out BEFORE you buy. Situation-depending, you'll need to know this. Sorry, the only 'references' I can give you in the NYC area is to check the YELLOW PAGES for Custom Door Pros (found under DOORS.) My best to ya and hope this helps.