I've experimented with a number of things over the years and, frankly, every method I've tried worked, none noticeably better or worse than any other.
Eggshells--rinsed, baked and ground to a powder can be integrated into any soft food, mixed into the substrate or just dusted onto blanched veggies. Or mixed into the substrate.
Oyster shells--I generally just bash one with a hammer and drop the pieces somewhere inconspicuous. I've also known folks to mix pieces in with their larger filter media or put a couple broken shells in their HOB.
Cuttlebone--same as oyster shells, though I've noticed snails will tend to congregate on the cuttlebone pieces itself when it's in the tank and I've never seen them do that with oyster shells.
Super expensive japanese powder that, once translated, proved to be nothing but finely powdered oyster shells and a smidge of dried seaweed.