No source for something ready-made. Our lead carpenter made me a 'box', mirrored the back, and put an electrical outlet in it. The 'door' is a decorative mirror hinged onto the box; it fits flush to the wall.

I just went through this same exercise, an din the end, fixed up the old cabinet. Alternative would have been to just cover the opening with a larger surface mounted cabinet or mirror. You can search by, more or less, size on the Home Depot and the Lowe's sites.

Or try this site: http://www.build.com/medicine-cabinets/c108513?F19797=recessed

This one is 13.5 inches wide. It has a polished edge instead of a metal banded edge, though. The inside is plastic, with plastic shelves. It has a 17.5 inch high cabinet, so you will have to make your hole smaller. It is also only 3" deep.

http://www.cabinetsmedicine.com/product/broan-nutone-b7233-focus-single-door-recessed-medicine-cabinet-2-shelves

I would suggest that if you really want to replace the one you have with an actual medicine cabinet instead of using the methods others have suggested, you take it out and feel along the edges to see if you can easily widen your hole. Most cabinets are at least 14" wide. That half inch opens up a whole world of small cabinets to you. If the previous hole is not boxed with wood, you might be able to easily widen and lengthen it to suit a different cabinet.

The sellers are not dumb. They want you to buy the cabinet to find out the cut out dimensions.