I used to, but they are expensive and I keep costs as low as I can by using/recycling what I happen to have around or reusing what I can for different purposes. (The whole permaculture thing). But if I have to and am unable to find an alternative I will just go back to using oysters.
You can boost their calcium intake with a number of alternatives; High on the list would be ground/crushed egg shells. Also on the list are things like seafood (sardines come to mind since they're cheap), leafy greens (not iceberg lettuce), like collard greens, spinach, turnip greens, kale, mustard greens, beet greens, bok choy, okra, swiss chard, etc., legumes (cooked only), dried fruit, dairy products like yogurt or shredded cheese. Many of the items on this list are pricy, but if you've got an egg bound hen, in an emergency, they'll all provide a great calcium boost which may ultimately save a hens life.
None of the feed stores offer oyster shell in my country, apparently it costs too much; but they DO offer ground up eggshell by the pound, can buy as much or as little as I like, so I buy that by the bagful plus feed them back their own eggshells, crushed.
Of course!
I also sprinkle some in their layer feed
This is what I do, also. I switched after I realized that my Brahma pullet was probably not getting enough protein. She's still growing afterall.
Yes, it was sperate.
