Monoxide is incredibly cheap to produce with a crappy farm truck or old tractor. You doing need to distill or concentrate anything, just a hose and the exhaust pipe and a couple hours of fuel for idling.
We used it to gas a nest of rats that had settled in under a grain bin floor. Only a couple rats popped out and they were dazed, the dogs quickly snacked them up. The rest expired rapidly.
A chicken barn is big and drafty but you could just use multiple tractors or detune them on purpose. Any engine running rich produces a lot of CO.