We converted a 6x4 shed for our hens, we got free out of a supermarket notice board, Try your free-adds or local classified, you never know what you might find,
i wouldn't buy of ebay, my Gran bought one and it was well to put it bluntly s***, cost a bomb and fell apart after 3days of use, and the ebay seller was extremely rude about it and blamed her!
I feel that housing is one thing that you should go and look at before buying, you could try your local Woodcraft centre