I had the same problem in my house. I decided to locate my enclosure inside my coat closet in the entry. Unfortunately, the cavity was about 1/2" short of the standard width (the stud seems to be a little closer than standard, and is either a crooked piece of wood or went in not exactly straight).. which I only discovered after I cut away the drywall.
I found that almost every enclosure assumed 16" center-to-center. I did find a couple of enclosures that were 12" wide, though: Data Comm and Cooper Wiring make 12"x28" and 12"x44" (they're sold under these two brands but are AFAICT the same unit). Unfortunately, being less wide than the more common enclosures (e.g. Leviton, OnQ, etc) means that the Data Comm / Cooper enclosure isn't wide enough to have two columns of modules side by side.
I ended up buying a Data Comm 12"x28" enclosure and was planning to put that into the closet. Unfortunately a cpl weeks later I had some scope creep and now I'm relatively certain that the 28" enclosure won't be big enough. Now I'm debating getting the 12"x44" or notching the stud to make a standard-sized enclosure fit (any other alternatives, short of opening up more drywall and putting in another stud, and having to patch/paint it all?). I'm guessing the wall isn't load bearing (it doesn't look like it), but I'd like to be certain.. is there any definitive way to tell?
Also, if I were to notch the stud to make an extra 1/2" or so of room, what's the easiest way to get a clean cut?