I buy bagged salad mixes on sale, plus I hang a cabbage from a shepherd's crook in the run for them to jump up to peck at it. They tear off pieces and the cabbage is gone that same day (but I do have a large flock).
We found 2 things they like, one by accident the other by others here.
1. We bought them whole oats during the summer which they did not like, so I threw them out in the garden and they grew very nice and we cut what we want each day, the oat grass is not effected by the cold nor snow, so they will have the grass all winter.
2. Alfalfa cubes, soaked in hot water until moist and mixed into feed or yogurt or cottage cheese or whatever you want, seems to be a good green for them to eat and goes a very long way.
I give my girls canned peas and carrots every morning in their oatmeal (made with vit D milk). In their suet feeders I put Broccoli, tomatoes, cukes, lettuce, bananas, or apples They get the peas/carrots/oatmeal every morning now that it's chilly outside and one item in their suet feeder.
I bought a 50lb bag from our local feed store like the ones for horses, cost about $14 but I only use 3 cubes a couple times a week for my 13 girls so it will last a very long time and it stores just fine so no issue with it going bad.
Kale. You can buy it in a bunch at the grocery store, and they seem to love it. It only costs about $2.00, or even less per bunch. I also like to give them a whole cabbage. And apples (I know they're not greens....)