This is where we got ours: https://thedreamweddingshop.com/. I was very happy with the quality of the product. We had our names and anniversary included on it and it's currently displayed in our living room.
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Basically the bride and groom combine their two containers of sand into one symbolizing the meshing of their families and lives into one indistinguishable unit, unable to be separated into their original states. It's a unity ceremony, one of many varieties. Some people also use colored sand to create a more visually stimulating metaphor.