Here's a good list of food for people to get selenium in their diet. You could probably garner some ideas for chickens from the list.
https://www.healthline.com/health/selenium-foods#chicken
Good idea.
Appears the answer is "meat scraps" (those chicken feed recipes pre-1950), cowpeas in the summer and cruciferous veggies (spinach, kale, etc) in the winter. and of course the beans (cowpeas ) and the greens need to be cooked. Got it.