And a darned good one. Rushing won't do, and you've got to have someone who understands how to match your foot with the boot that will work best. Once in the store, find a salesperson and explain what you're looking for. If you have any particular fit problems, tell him or her what they are. Then, have the salesperson measure BOTH feet, not just one.
Some people can vary in foot size by a half-size or more.
Put your socks on, and start trying on boots. The rule of thumb is that a hiking boot should be sized about a half-size larger than, say, your favorite running shoes. But that isn't always the case. In the store, walk around in the boots. Expect some stiffness, but don't tolerate any obvious pressure points or pinches.