if you can find a solid 42" door you can take hte hinges off it and rip 3/16" off all four edges then reasseble it.
another thing you can do is go with the 40" and install a half jamb with 3/4" stock that the door tucks behind which will hide the space
another thing you can do is go with the 40" and install a half jamb with 3/4" stock that the door tucks behind which will hide the space