Storm Door modification
Your first option is to see if there is any addition to the door jamb that can be removed to give you a standard width.  Sometimes there is additional molding added that may appear to be one piece, but it can be removed.  If it's not trim, you may even be able to modify the jamb with a router to give you a little extra size on either edge (more advanced, but cost effective if you have the tools).

If the jamb is an odd size, then you can get odd sized storm doors.  They might not have them in stock, but sizes like 34" can be ordered.  Get the closest size available and add trim to make it fit.

As a random example, this Andersen door can be ordered at 33", 34" or 36" (not recommending this door, just a quick example of odd sizes).  If you call around, you might even be able to find a 35" door.  There are shops that specialize in doors and windows and they might be your best bet.