It might be worth trying Jack's Stables in Fore Street. It used to be free if you collect, but I think they charge to deliver. Or there is a large stables on the main road near Mount Vernon Hospital. And I bet there's something in Harefield. But, as Eristic says, transport is the expensive bit. (All stables have plenty of the stuff to get rid of! We're lucky - we have a stables on-site, so "transport" means a wheelbarrow.)