info on oxalates in spinach and other foods that may inhibit absorption, but seem to be less of a problem if calcium intake is relatively high:
https://blog.algaecal.com/oxalic-acid/#:~:text=Oxalates%20can%20bind%20to%20minerals%20and%20prevent%20them,daily%20amount%20of%20calcium%2C%20this%20effect%20isn%E2%80%99t%20seen.

Dairy is a good source of calcium if one can digest it. There is no calcium loss from consuming dairy, you just might not incorporate all of it if you are allergic or lactose intolerant. It is one of the highest sources of calcium, best consumed as fermented dairy like yogurt, kefir, and better yet, kefir from goat or sheep as those are more digestable than cow dairy. Dairy that is A2 is also more digestable. Greens have calcium but as windyshores mentioned, some are high in oxylates, particularly spinach, one of the highest in oxylates, which inhibits the absorption of calcium. Tofu is high in calcium but only if it is coagulated with calcium sulfate so best to check the label as some tofu is coagulated with magnesium sulfate in which case there is no calcium content. Fortified products are fine to consume for your calcium intake but it's important to know that it is the same as taking a calcium supplement as these products do not naturally contain calcium. In addition, most of those products use calcium carbonate as it is the cheapest source of calcium but it is also the least absorbable form of calcium. Lots to consider when figuring out our calcium needs and sources. Whole foods are always the best option from sources that best fit your lifestyle and health requirements with supplementation as necessary to fill in the gaps.