Here’s a good interactive hardiness zone map from the US Department of Agriculture.
It lets you put in your zip code and get detailed information about your area, including the sub-designation for your hardiness zone. For example, in Blacksburg where I live, I’m in zone 6a, where someone in Roanoke may be in zone 7a.
The zip code input is kind of hard to see; it’s located at the very top of the page.
The hardiness zones are based on the lowest average winter temperature for the area.