You can buy treats in a feed store such as meal worms, scratch or a flock block(huge, compressed block of grains for poultry). But you give plenty of treats to your chickens right at home.
My chickens love bread, canned & dry catfood(fed sparingly, usually for a protein boost), leftovers(noodle, sauce, cheese, meat,-I'm basically describing a recipe for spagetti!), yogurt, and scrambled eggs. Raw potato skins should be avoided though, because there's something in the raw skin they can't have(cooked skins are good though!). I've found if I have a chicken feeling under the weather (such as when one of mine had an egg partly crack inside her) who wouldn't eat or drink, scrambled eggs did the trick! After she ate those, she started drinking water which chickens vitally need.