Yorkshire Beehives
I attended the YBKA Conference at Burton Latimer College in Beverley about 7 weeks ago and saw the stand featuring hives being sold under the Yorkshire Beehives name. They appeared more solidly constructed and reasonably priced compared to the larger firms. The proprietor appears to be a one man band with a pride in his products so perhaps deserves to be supported - I suppose that he could produce a 14 X 12 broodbox for you. You can Google Yorkshire Beehives
Otherwise, you could try making your own - I've been making Nat broodboxes from 18mm ply very successfully and simply - a larger broodbox should not be any more difficult.