I actually saw some in the fall Eddie Bauer catalog yesterday. They have them in several of their styles. May be too expensive if your legs get thin too fast. You could walk right out of them.
If you are looking for some place local... Off Broadway Shoes and Nordstrom have wide calf boots. I thought I had lost enough weight for some regular boots... still not there yet. I was so disappointed.
Aeresole's usually carries plus size calf boots. I have a pair in brown and black. I have found them at reasonable prices at Rack Room and Famous Footwear. Of course I bought mine super cheap at end of the year clearance sales :-) On sale though they are reasonably priced.