I take New Chapter Bone Strength calcium supplements. The calcium is plant based, sourced from algae which is naturally high in calcium. The pills also contain vitamins D, K1 and K2, which directs the calcium to your bones. The daily dose is divided into 3 pills, so they aren't huge and you can space them out during the day. They have never upset my stomach, even when taken w/o food. I purchase them on Amazon.