The best replacement for oyster shell is calcium carbonate.

The best alternatives to oyster shell are rated as following:
★★★★★  calcium carbonate
★★★★  calcium citrate
★★★  grit
★  chicken grit
  carbonate

Your alternative for oyster shell is not listed? Suggest!
Speaking about oyster shell, one probably shouldn't forget about calcium.

Other search terms related to oyster shell and their ratings are:
★★★★★ calcium
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oyster shell
calcium carbonate
calcium citrate