Speaking as someone who is still on a search for the right boots for a 12.5" calf and a bad foot. . .It's trial and error. Nordstrom's does have a way to search for boots that have a narrow calf circumference (click on "Boot Shop" and choose "narrow calf" from the drop-down menu), but it's hit or miss. You can do some research there and then go to Zappos, which has better descriptions, photos, and reviews.
One thing I've tried is to eyeball the photos of boots on Zappos--if the calf looks really narrow in the image, I check the numbers. Sometimes I'm right.
The labels that tend toward a narrow calf are the more expensive ones. If you have unlimited funds, Prada, Robert Clegerie, Delman, Aquitalia, and Stuart Weitzman are narrow-calf brands. If you're in the same price range as most of us, I'd recommend looking at labels made in Spain, like Martinez Valero or Jeffrey Campbell. Rockport has a narrow-calf boot--but no zipper, so getting it off requires major effort!
Duo Boots, a UK company that sells online, makes custom boots and can make most of their designs with a narrow calf (again, not cheap, but may be worth it).
Keep in mind that if you find a pair of boots you like at a reasonable price, you can take them to a cobbler for an estimate of the cost of having them taken in around the calf. It's not cheap (I hear it runs around $100), but if your cobbler is skilled, you've just turned okay boots into fab boots.
Hope that helps. Let me know if you find anything fab and comfy with a narrow calf--best of luck!