The best replacement for calcium citrate is calcium carbonate.

The best alternatives to calcium citrate are rated as following:
★★★★★  calcium carbonate
★★★★★  carbonate
★★★  calcium
★★★  caltrate
★★★  calcium caltrate
★★  coral calcium
★★  elemental calcium
★  oyster shell
★  calcium phosphate
★  malate
★  lactate
★  calcium magnesium
★  calcium lactate
★  calcium gluconate
★  calcium orotate
★  calcium acetate
★  calcium carbonate absorption
  calcium malate
  calcium citrate malate
  tricalcium phosphate
  calcium from algae
  carbonate absorption

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calcium citrate
calcium carbonate