All those in the cabbage and mustard families, especially collards and kale. Sesame seeds have about 1600 mg per cup. I add sesame seeds to their feed. They love them and it seems to add to the thickness of their eggshells. It is also cheap and available throughout the year. Most white beans, seeds and greens, including grass, have usuable amounts of calcium. Figs, peaches and apricots have some, too. Mealworms and soldierflies and most hardshelled insects have calcium.
There is also a relationship between phosphorus, salt and protein and calcium uptake/retention. Too much salt and protein means the chickens will have to consume more calcium. Too little or too much phosphorus, the same thing.
I give mine access to greens, insects, fruits, calcium and phosphorus rich grains and seeds in addition to free oyster shell. They have never been eggbound and have hard shells.