I'd Google for pigeon leg tags as they seem to come with various information on them. Another way to go of course would be to get a unique colour for your chickens and alert the local vet in case yours were ever taken in. Good luck.
my chooks: Sage&Onion, Roger the randy cockerel, Squawkbox, Bonnie 'N' Clyde, Pancakes, Custard, Omelette & EggNog. Plus four garden cats and another half dozen feral ones that I feed. And a nursery of pipistrelle bats in the verandah roof ..