You don't need oyster shell if you feed layer. The 3% min calcium is far more than they need.
You can supply small stones in winter to birds. Grab a small bucket from driveway pre frost. It doesn't take much and most winters there is a January thaw where they can dig at the ground anyway. If...
I just toss a few handfuls (depending how many layers you have) once a week. By shell thickness you'll know to use more or less. I don't use a layer feed ever. Entire flock is on non medicated crumbles when raising chicks then entire flock is on turkey finisher pellets when youngest bird are...