Craigslist can be a good place to search for coops. Keep in mind that it doesn't have to be built specifically for chickens. In some cases it can be much cheaper to buy something like a small wooden storage shed and install a window, a few roosts, and some nesting boxes (which are easy tasks even for those like you and me who are not architecturally inclined).
Welcome to BYC - thanks for joining us. I totally agree with QMisha - sheds are often cheaper, better quality than pre-fab coops and are flexible enough for changes to be made - very important when it comes to chicken maths