Don't urban people keep exotic birds indoors? Parrots, cockatoos, African finches, etc. So you might try asking the places where those are sold, would that be pet shops? And looking for exotic bird groups in your area on Facebook, and asking them who they take their birds to when they're sick. I've not got a vet yet but the two I found that may take chickens are actually in urban areas, generally two hours from me, and their practice seems based on the exotic birds.I've called around with the sincere hope that exotic meant chicken too. No luck. My dogs' vet recommended one of their competitors, and that practice recently lost their vet that treated chickens.
I live in too urban of an area to have access to a farm animal vet.