Random, as for calcium, I usually buy these little feeder "blocks" from a major retail store, just make sure that calcium is one of the very first ingredients listed, ingredients are listed in the order of content. People have suggested cuttle bone to me and it might work but once I got something to make the bone sink my snails said they were yucky and didn't want anything to do with them, but you might have different results. People have also suggested calcium vitamins (made for people) just try to make sure there aren't considerable amounts of anything that could hurt your snails (internet research is the only way I know to do this). I believe the reason I didn't buy any vitamins to try on my snails is because the vitamins I could find locally had some metals in them that I wasn't sure about. Also, if your snaily has any large areas of shell that need immediate attention, and are not too close to the opening of his shell I have used a product to patch one of my snails called Water Weld (made by JB Weld). You'll need patience and your snail will too but this works good and after a few minutes (read directions) you can put him (or her) back in the tank and the Water Weld will cure under water. I hope everything turns out okay for you.
