Check Craigslist. There are some listed on there in different sizes and such. We built our own from plans we purchased for $20. We already had almost every tool necessary though. Every time I do a project I justify to myself the "savings" of doing it myself will cover the cost of whatever tool I want at the time. That's how we've ended up with all the saws, tools, etc.
We just got started this spring. In hindsight I kind of wish we'd have just bought one of the different used coops that pop up from time to time on CL. People on this forum talk about "chicken math" which is code for "chicken addiction". We thought we'd want to have 6-8 laying birds to provide eggs for our 4 person family. So that's the size coop we built. Now we've got 20, and will probably add more (broilers) from time to time to stock the freezer. If we'd gone with a used coop we could have learned, then built what we really need once we knew what we'd really want. If that makes sense.
No, my current coop isn't for sale. The in-laws have called dibs on it if we decide to go ahead and upgrade. ;-)
I recommend "The Small-Scale Poultry Flock" by Harvey Ussery. I've been reading it recently and it answers outstanding questions I'd had and has a ton of ideas I may try out in the future.
Good luck with your chicken adventure!
