You could order one custom door from a cabinetmaker that would fit. We just had a pantry built this week. We took some space from the closet and turned it into a pantry. Our cabinet maker is making a door to fix our odd space. I think they can make doors in any size, or so it seems.

You might be able to just have doors made and then add shelving. Depends on what the fridge cabinet is like.

It really depends on the space and your existing cabinets. Do you have wall on either side of the space, or is it cabinets on both sides? If cabinets, then I agree with seeing about getting something custom made to slide into the space and match what you already have. You could consider IKEA pantry units with all the nifty oull-outs and then have custom doors made.

Fridge spaces are usually 36" wide or maybe 30" wide if your house is more than about 40 years old. If you have wall on at least one side (fridge on the end of the cabinet run) and 36", you could have another wall built (with 2 x 4 framing and drywall this will consume about 4-3/4" of space) and possibly have room for a prehung 30" door. You could finish the interior however you'd like.