Zappos carries a broad range of wide calf boot retailers. With their search function, you can narrow down choices by calf circumference as well as color, textile, heel height, and size.
Torrid is great for trendy wide calf knee and over the knee boots at budget-friendly pricesOffering wide (2" wider than standard) and extra wide (4" wider) in most of their styles, you're likely to find a pair that fits great!
Jessica London carries an extensive selection wide calf boots at wallet-friendly prices. Each style comes in a range of shoe widths and shares the boot shaft circumference.