i second craigslist. i posted and within a day had a guy offer me a really nice 4' x 3.5' x 3' dog house that's fully insulated for $25. make sure you list dog houses and rabbit hutches when you're saying you're looking for something. a lot of times people will read the ad and think "oh, i don't have a coop" and forget that they have an old something-or-other in the backyard.
i have a dumb question i keep reading about craig list - what is it and how do i find things on it? i am also looking for a coop/shed for my girls for the winter months