Go to someplace like Habitat's recycled building material store and buy a second-hand door.
Can't; they're not standard sized. We had this same problem when we needed to replace our back doors (two full mullioned french doors) after our dog ate through them (long story, but true); we couldn't find doors that would fit; they were apparently custom made (the doors exit from an addition to the original house that was put on before we moved in). It was going to cost over a grand to have them replaced, so we decided to wait until we could afford new windows and insulation put in.
We finally had that done last year, but having that door custom replaced was still going to be an expensive proposition, plus we wanted a way to be able to open the windows when it was cool out (before, we opened the whole door, which let in insects). That meant we had to buy new standard doors, and it was cheaper to do that than having the french doors custom made to fit - even though they had to redo the opening to make the new frame fit. To have this same process done to my shop would be more money than I want to spend right now.
Thanks for the suggestion, though!
