Kenneth Cole have had a really good range of boots this season that have all been quite wide fitting, but there are very few left now anywhere i would imagine, the season is pretty much over and the new spring lines are already in the stores.
Evans are good and I got a pair from Wallis a couple of weeks ago that fitted well around the calves (not particularly big myself - a size 14 - but all boots seem to be designed with stick insects in mind!)
I've also heard good things about Duo -can pick lots of different calf sizes, but they cost a bit more than high street:
Google Additions, and they have up to the knee boots from �15 in mock suede. They, and Simply Be often sell over the knee boots as well, at really low prices. Better still, Littlewoods are selling them off for �5 a pair, but it does depend on how wide you want them, and what style. Viva La Diva still have some in stock.
duoboots.com make boots according to your individual measurements. Send them your calf width, ankle width and measurement to your knee and chose a style and they'll make them for you
I have some from Evans and also some lace up ones (Mr Spudqueen finds them sexy!) from Clarkes. Because they lace up the front (but have a zip up the size) I could alter the width of the lacing up so they fit!