One of the best sources I have found for boots for narrow calves is Zappos. They have such a fantastic selection and great prices to boot (no pun intended!).
Look for boots made from fabric that will allow for some give and take - stretch suede is a fabulous choice because it offers just that, as well as being very luxurious (and sexy!)
Another great option is a boot with buckle trim. Not only is it stylish, but the buckles serve the practical purpose of being able to customize the fit to your calf. Side-tie or front tie boots offer the same stylish benefit.
And finally, this is an option that worked wonderfully for a fabulously chic and stylish friend: she found a pair of boots in Italy that she absolutely fell in love with. They zipped up the side, but unfortunately were too large on her calves (sound familiar?) The clerk at the shop told her that she could take them to a cobbler and he could remove and reinstall the zipper, which would take an inch or more off the circumferance. My friend ended up buying the boots in Italy and having the zippers replaced by an equestrian boot repair shop near where she lives. She said it was well worth the added cost because the Italian leather boots are such a classic style she will wear them for years and years. So it actually ended up being an investment!