I found the following boots that measure 18 inches or more for the plus size gal.
Ros Hommerson Women's Step Wide Calf Boot (Black 5.0 M)
Leather dress boots with full side zipper , cushioned insole and 18 inch calf circumfrence
A leather riding boot leather with an extra wide calf measurment ranging from
17 1/2" (for a size 6) to 19 3/4" (for a size 12). Has cushioned insole ,full side zipper and elastic gore for extra stretch.
Channel Extra Wide Calf Boot Black Leather 7.5M
A dress boot available in black or chocolate brown that fits calf sizes ranging from 17 1/2" (for a size 6) to 19 1/2" (for a size 13). It has full side zip, elastic gore, soft inner lining and a comfortable 2 1/2 inch heel.
Ilana Brown Extra Wide Calf Boot 7WW
A knee-high stretch boot with pointed toe and sharp heel that has extra wide calf circumference from approx. 17" (maximum for a size 7) to 21" (maximum for a size 12). Has a full length zipper.
Hayley Brown Extra Wide Calf Boot 9W
Jessica London Plus Size Adedaline Wide Calf Boot (BLACK,9 M)
Leather- like stretch boot that accomodates calf sizes from 17" for size 7 to 20" for size 12. Come in 4 Colours as featured in the picture above.
Mid length riding boots whose 16.5 inch boot opening can stretch a further 2 inches to 18.5. Has adjustable buckles, flat heel and is available in Black or Brown.
Avenue Plus Size Roselyn Buckled Riding Boot (Wide Width) (BROWN,9 W)