We feed their shells back to them instead of oyster shell, mostly. By that I mean we started off with oyster shell, but we have lots of eggshells, and it seemed like a waste to just throw them in the compost pile. Actually, they raid our compost pile everytime they free range (nearly every day) so they'd get the shells anyway.
We collect them in a tupperware bin, and once a week I put them on a cookie sheet and bake them at 275 for 30 minutes. They come out very dry and brittle. I put them in a metal bowl and crush them down with the back of a heavy plastic ladle. I crush them down to about the size of sugar granules, takes about 3 minutes.
BTW - you can also crush other crustasean shells you might have leftover, instead of buying oyster shells. Clams, mussels, scallops, lobster, shrimp (I think), etc...