I actually have not purchased oyster shell, as I reuse egg shells instead. I started saving them up months before I had any laying hens. After cleaning out the egg, I put the shells on a cookie sheet. Whenever I use the oven, after I've turned it off, I put the cookie sheet in the oven so that the residual heat dries them out without burning them. Once dry, I put them in a glass jar and use a heavy ice-cream scoop to crush them. (I used to use the food processor but then someone mentioned that the plastic bowl of the food processor would be torn up by the egg shells so that's when I started using a glass jar). I keep crushing until each piece is about 1/4" across, then store them until needed. When my eldest hen started showing signs of maturity, I took a yogurt container outside and nailed it to a deck post, then filled it with the crushed egg shells. I've watched my hen go to it to eat and have even had to refill it once.
I did have some fear that this would teach her to eat her own eggs but so far that has not been the case. I'm hoping that by "cooking" them and crushing them, they look different enough from the eggs she lays that it won't occur to her. She started laying two days after Christmas and so far has not touched her own eggs.
I know oyster shell is not a huge expense but this way it is one less thing to have to buy and I am able to reuse something I was going to be throwing away otherwise.