My profile picture shows some flowers that would go well with calla lilies. I am not a floral designer so I am not sure what would match the best….for a better answer, I would try posting your question in gardening.
In the image link, the flowers appear to be pink and cerise roses, pink peonies, and pink cockscomb as well as mini pink calla lilies.
sweet heart roses, carnations with a touch of baby’s breath.
I would say just add a touch of babys breath to the bouquet because calla lillies really stand out on their own, no need to match them with anything else.
Look how lovely this one looks in simple white with the greens:http://www.links2love.com/weddings/white_bridal_bouquets.htm
White is always classic.
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have fun.
Small white or pink carnations
Mini Anthuriums. You have a modern shape with the callas, so I think mini Anthuriums would suit the simple form of the callas. They are available in an array of colours from reds, to creams and pinks, white and green. A little vine wrapped around the outside would go well too. Because of the shape of the callas, I would possibly go for more of an arm spray shaped bouquet (one that you hold up your forearm, as a posy may hide the beautiful shape of your callas. I have pics of some bouquets and arrangements with green anthuriums in them, and also some different shaped bouquets on my site if it would be of any help to inspire you.