Fortunately, a few boot makers DO turn out boots available in narrow widths. But darned few - most companies that bother with widths crank out a regular and a wide. But Kayland, an Italian maker of good-quality boots with a U.S. distributor, makes narrow boots. Kayland's Traction K ($215), a nice-looking medium-heavy backpacking boot, is available in narrow. Vasque makes some boots in narrow as well - the Sundowner ($175) is a classic backpacking boot that may fit you well.
Otherwise, it's going to take an experienced boot fitter who knows the relative widths and volume of supposedly "medium" boots to find the right one for you. And that should be possible. Find a good backpacking store that has a wide assortment of boots - if Kayland or Vasque, so much the better - explain your problem, and let someone work on you for a while. You might actually be able to use a "regular" boot through an astute combination of socks, after-market insoles, and plain good fit. Also, some boots just seem to fit a wider range of people than others. Into this category I would put L.L. Bean's Leather Cresta Hiker ($165) and several boots from Montrail, including the Vercors ($190) which fit me well even though I have a moderately narrow foot.