Contact your local home repair companies. They are often tearing out materials in order to rebuild. Often perfectly good materials which would be great for a coop build. My SIL is always coming home with salvage materials. Everything from plywood to 2 x 4's, stair stringers, paving blocks...
Do you have the tools? At the very least, you will need a good drill, and a skill saw. I much prefer to use a chop saw for most of my cuts.
What is your basic location? That will help folks to give climate appropriate advice. While you already have your coop drawn out, I suggest that you...