Actually, you
can get recycled paint, which is made out of "leftover" paints mixed together. Here it is made by the solid waste district, but they sell it in some local retail outlets. Somehow they make decent colors out of it, too. A reused building materials store will also have used hinges. I bought used hinges for my chicken tractor, and also used latches for the doors. Used caulking - now
that's a good one! Occasionally the reused building materials store will have a tube that someone bought and then didn't use.
My coop is built into the corner of a 100% reused 100+ year old barn (reusing 2 of the existing walls). Inside the barn, some of the new walls are made out of some hollow-core bifold doors that I had from rehabbing my tenant's unit. This worked very well, as the doors were like large sheets of lumber. I bought very little for my coop - most of it was built from odds and ends I had lying around the barn.
For roofing, in addition to tackyrama's salvage yard idea, you can get the "cover sheet" from a delivery of metal roofing from a new building materials center. The cover sheet is scratched up by the strapping around the bundle of roofing, so they can't sell it for full price. I dickered with my building materials guys for a good price, for the roof of my chicken tractor.
I'm very excited that a new reused building materials store is opening near me soon. Currently I have to drive 40-50 miles to get to one (combined with other errands, of course).
I really liked the idea of the kitchen cabinet coop, too, that someone posted about recently. It looked very nice when all painted the same color.