Here's where a tape measure can really help. . .If you measure the distance from the bottom of your heel to the back of your knee, wherever you'd like your boots to hit, that's your shaft length. If you measure the circumference of the widest part of your calf, that's your calf circumference.
Zappos and Nordstrom's both give shaft and circumference measurements on their websites. Having both of those measurements will let you weed out the boots that aren't going to work--you can save yourself a lot of time and effort that way.
This month's Lucky Magazine featured boots that are designed to hit just above the knee--no "pirate looking" ones among them. Over-the-knee boots usually have a 19-20" shaft, which may give you the height you're looking for. I don't have the issue at hand, but it shouldn't be difficult to find. Lucky usually features a wide range of price points, too, so it's not hard to find something within your budget.