I got mine from Albany Pet, who used to advertise on Ebay, they do have a website, although the one I got is not advertised, maybe you could contact them and ask what they could supply. mine is 5ftx6ftx29inches and is fab.
I also got one for the grass from zooplus
http://www.zooplus.co.uk/shop/rodents/cages/pens_fencing/pens_with_racks/35638
I had the small one, which was good, but now I have two of the big ones to go around my garden so they can semi free range, when I have time to supervise, or to put together as two, if I don't have as much time to keep an eye on them