There are two repair possibilities. First, there are a number of chemicals and epoxies that can stabilize and repair rotted wood. They are used extensively in restoration projects, like rebuilding architectural features of old homes, porch posts, window trim, etc..... this avoids the need for replacement parts, that would have to be custom made. Second the door frame could be removed and rebuilt. This would involve disassembling the frame and replacing the jamb "leg". The entire part can be ordered from your local lumberyard.This would requires some pretty advanced skills however.
The only hope for the door is digging the rot out and filling the area with epoxy, or other filler. The wooden edge you are looking at is called a stile, it is deeply embedded in that style of door construction. Frequently the steel skin is tucked into the edge of the stile, then the steel (or fiberglass skin) is glued to the stile and other framing members, then the entire hollow door is filled with blown in place foam. This usually eliminates any possibility of replacing the stile.
The other possibility would involve dealing with a real sharp local lumberyard or local millwork supplier. All steel/fiberglass doors are built in millwork shops that stock the "blanks" required to start the process. Your blanks were probably just smooth steel, windowless doors that were routed out to create hinge slots, a bore for the lockset and latch, and a big rectangular opening for the window glazing. You might find a supplier who would sell you the precut door, minus the glazing. It's just a matter of swapping the hinges, lock and glass to the new one.
