I have and use oyster shell, and they do go to it slowly. I give the oyster shell while I save up on egg shell. Once I get around 4 cups crushed by about the spring time and then to start the new egg season off I give them the egg shell.
I find that since they don't lay much in the winter the oyster shell will keep them with enough calcium in the cold months.
Since I have been doing this when I give them the crushed egg shell in the spring I am blessed with 19 eggs a day from 20 hens for the first month or 2 before it slows down because of molting.
I always clean the membrain out (thin skin around inside of shell)when I wash them after cracking leave them to dry out over night on a paper towel, throw them all in a 1 gallon clean bucket. Then every 2-3 weeks I crush the shells to the same size as the oyster shell and store in dry container.
In the spring I mix equil parts of egg shell and oyster shell and give to my girls.
Wow my egg collection goes into high gear with in 1 week.
***it is best to clean the thin skin out of the shell before drying because that thin skin could harbor the chickens breathing if it gose across the throat.
If you can get the shells from a bakery, I would say ...YES, go for it.