Cuttlebone is available at pet stores and from suppliers of bird products.  Besides being a rich source of calcium, it provides your bird with a plaything, as they enjoy nibbling at it.  This also helps keep their beak in shape to some extent.

Hang it in the cage with the soft side facing in.  You can purchase special clips to hold the cuttlebone, or you can use any utility clip.  Florist's wire can also be wrapped around it and then secured to the cage. 

If your bird won't eat cuttlebone, occasionally crush some and sprinkle it on their food.

Cuttlebone comes from a cuttlefish. The cuttlefish is not a fish, but is related to both the squid and the octopus.  It has an internal shell, called the cuttlebone.  The cuttlebone is made from calcium carbonate and helps control the buoyancy of the cuttlefish. 

This is a photo of a cuttlefish.  They are an interesting creature, having a phenomenal ability to quickly modify their color pattern to match their background and to communicate.  They are sometimes called the "chameleons of the sea". 

Cuttlefish only live about two years, and die after spawning.  Beachcombers will often find the cuttlebone washed up on shore, but because of the danger of pollution, it's not advisable to use those ones for your bird.

Without the cuttlefish, many great works of art and literature would not exist.  The dark brown ink of the cuttlefish, called sepia, was once used extensively for writing and drawing. 

Cuttlefish are caught by commercial fisherman, and are used for food (check out some recipes on the internet), as well as for the cuttlebone.