you could also try sites like ad-trader or friday ads, they sometimes have the 'free to collector' ads for greenhouses. Is there a 'gumtree' in your area? I picked up one from here. Ask everyone you know and get them to ask everyone they know. If you are patient and vigilant, they do turn up.
As previously mentioned, get the biggest one you can, trust me you first look at it and think you will never fill it and two months down the line you will back on the www looking for another one
