Availability is the big thing, if eggshells are available then use them, my wife is alergic to eggs so i use pulverized dolomite(CaMg(CO3)2). The thing to remember is when buying lime to use get pulverized limestone/dolimite not hydrated or slacked lyme, those will burn your worms

"Smoke 'em if You Got 'em" if not purchase powdered limestone. I feel the number of eggshells my family produces equals the exact amount of lime my worm bin needs.

"Smoke 'em if You Got 'em" is a good thought process for worming. Not to go out and buy extra stuff for them. Using the leftovers from what you naturallly process be that food scraps, egg shells, etc. I do occasionally stop by starbucks waste bins and find their bagged used coffee, its free. I use a little in worm bins and on my outside plants, nothing wasted. Im trying to be natural concerning what goes in the bin. There has been a lot of talk about adding rock dust which I dont , another item which would have to be bought. If you look at a compost pile whats in that most likely not rock dust , maybe some straw, hay, grass clippings and whatever else makes its way there. But I bet you wont go out and buy some rock dust for it