From my experience most brands consider over 15" calf circumference to be "wide calf". This varies a bit by brand, though. I'm just in that 15-15.5" range myself.
Endless provides a way to search by calf circumference either by selecting "wide calf" or by selecting calf circumference over 15.5".
Zappos allows you to search by a very specific calf circumference range for boots (for example, 19-19.75".)
Duo Boots is one retailer that provides a very large range of calf circumferences to choose from.
http://www.widewidths.com/ specializes specifically in wide width boots and offers a variety of width ranges to select from.
