Good sources of calcium for your hermies are:

Calcium powder

used with reptiles and amphibians, also great for land hermit crabs

Common brands are RepCal, StickyTongue and

Coral Sand

Attractive and natural source of calcium that hermit crabs love.

Used either as a substrate or a calcium supplement, coral sand comes in many different grades, from fine, medium, coarse and actual pieces of broken coral branches. You can put coral into the water dish for traction or crush it up as a quick calcium powder bonus for your hermit crabs.

Crab Exoskeletons

If you have access to a beach and you can find some crab exoskeletons (outer shell) you can rinse them in ocean water and offer them to your hermit crabs.

Cuttlefish/Cuttlebone

versatile calcium source

Grated, shredded, crushed, in pieces or whole. Can be added to food or served in a separate container (flat shell is suitable).

Crushed, Dried Egg Shell

Easy Alternative

if you do not have a source of cuttlebone or oyster shell, then some boiled, crushed, dried egg shells from a hen is a good alternative.

Crushed Oyster Shell

high calcium supplement

As found in pet stores and offered to birds. FMR suggests oyster shell as a good source of calcium for your hermies.

Spirulina

natural calcium source

Spirulina is high in calcium, protein, beta carotene and other nutrients which are needed in a healthy land hermit crab diet.