I feel your pain. My calves are "only" 13.5 but I have endured the eye-rolling of sales people as I tried on tall boots in stores. Sigh...
You might want to try this site: http://www.widewidths.com/
Even if you don't buy there, there is some interesting information about fitting boots there.
My solution so far is trifold:
1. One pair of Franco Sarto stretch boots. (Not leather) They are older so are not online now but they're the cousins of the boots Angie recently recommended: http://youlookfab.com/store/20.....etch-boot/
2. One pair of Clark's that have a stretch panel in the back: http://www.amazon.com/Clarks-W.....038;sr=1-7
3. A pair of Timberland boots that have an adjustable buckle at the top of the calf; on the widest hole it fits with jeans; on the narrowest it fits with tights.
In other words, adjustability is key to making it work.
Good luck, and let us know what you find!