Doors and Windows - Exterior door with cement threshold

Hi,
We have a 1957 brick house with built in cement thresholds/sill (our door swings over cement with just a weather strip to keep out drafts) and we want to replace the exterior doors with prehung doors. However, our door is not the standard 36x80, it's a little short. Does this mean we'll have to order custom doors? Or is there an alternative such as a prehung door without the built in sill? How would you replace a door in this situation?
Thank you.

Not nearly enough info. Single doors, patio doors, inswing, outswing, etc????? Specific opening sizes?? There are several companies that make replacement size doors that fiit existing older openings. Marvin(pricey) makes custom sized doors. Andersen has undersized stock doors.

Ok. Our front door is a single right inswing. The rough framework is 37 1/2" wide and 81" high. We are hoping to install a stock door because we are on a very limited budget so I'll look into Anderson. Thank you.

I am concerned about putting a new prehung door's threshold right over the existing cement one. Other door's we've replaced have a wooden sill that was covered by the pre-existing doors threshold. (Take out old prehung, replace with new prehung. Easy stuff.) And I want to explore any unknowns before I tear out our exhisting door and frame and have a hole in my house waiting for a last minute order to arrive that costs more than we want to spend. So any input on old door frames with cement threshold/sills is appreciated.