I think it might be really helpful for you if you had a calf measurement. If you need to be secret about it, is there a pair of skinny jeans that she wears that is somewhat snug around the calf without being stretched? That might give you a ballpark measurement to use when shopping.
A cobbler can reduce the calf on a boot, but not all boots lend themselves to this, and it can be somewhat expensive (my alteration cost $70). Also, you never know what kind of workmanship you're going to get. I would definitely exhaust my possibilities for a properly fitting boot off the shelf before going to a cobbler.
As far as where to find them, you just have to look, look, look. Angie (who runs this site) says that the more expensive boots usually have the slimmer calves.
There is a company in the UK called Duo that makes boots made-to-order for slim calves. Someone recently posted a thread about Duo on this forum, so it would be worth searching the archives. From what I have heard anecdotally, people are generally pleased with the Duo boots. They are rather expensive, but if you are really dealing with hard-to-fit calves (which I would consider to be less than 14"), they may be worth it.
Another place to look for good info is a blog run by a person who calls herself Little Contessa: http://littlecontessa.blogspot.com/.
Good luck!
