I have narrow calves and struggle too. It's awful. I think there are a lot of ladies with narrow calves who aren't finding what they need. It's frustrating.
As others have noted, things can be especially tricky if you have a larger foot size. Shoe manufacturers have decided that the larger the foot size, the larger the calf, which makes no sense. I have a size 6-6.5 foot and 12" calves.
I have boots that fit my calf from Aquatalia, Stuart Weitzman, and Cole Haan. Aquatalia and SW are especially expensive. I got my Aquatalias on sale from Amazon one year, when Amazon ran a special for 30% off all shoes around Christmastime. SWs can be found at Nordstrom Rack for about $450. Still pricey, but when you have small calves, you'll be hunting for high quality (which all of the above brands are) and less quantity.
I did have one pair of boots (by La Canadienne) taken in by a cobbler, and the result was so-so. I ordinarily recommend that people not do that, because the alteration is very difficult and often doesn't turn out well. Having said that, I do realize that even though the work is mediocre, it does give me a pair of riding boots to wear in the fall and on mild winter days, which I otherwise wouldn't have. So you can go this route, but be prepared to be underwhelmed. I was told by a cobbler that if you can find a boot that zips up the back as opposed to on the side, the alteration is much easier.
There is a brand of boot called Solemani that caters to narrow calves and is sold on Amazon. The boots come in 12" and 13" styles. I ordered a pair a few years ago. The fit in the calf was amazing! But there was too much space in the ankle, so I unfortunately had to send them back. But the quality and price point were good. I keep checking the Amazon Solemani offerings to see if they ever offer new styles, because I'd love to try them again.
For custom boots, there is Poppy and Barley, or Ted and Muffy. I think Ted and Muffy used to be Duo Boots. I have no experience with them.
I ordered a pair of over-the-knee boots from a brand called Bruno Premi last year, from Anthropologie. They fit perfectly in the calf, but they ran a little big for foot size, and I was already in the smallest size, so I returned them. I'm not sure if that was a one-off thing or if Bruno Premi boots are narrow in the calf in general.
When I shop the Anthropologie website, I have noticed that they almost always show the boots on the models' legs, which is so helpful. The legs of Anthro models are usually very skinny, and Anthro doesn't seem to Photoshop the boots to be smaller in the circumference, so you can guess how the boot will fit you by seeing how it fits the model. The other thing I like to do is see if Zappos has a video of the boot. Again, a model with usually skinny legs will be wearing the boot, so I can see about how it will fit me. The Zappos videos have saved me many times from ill-advised purchases!