According to Wikipedia, creamed coconut is "made from the unsweetened dehydrated fresh pulp of a mature coconut, ground to a semi-solid white creamy paste." It comes in a 200g solid block, wrapped on tight plastic. To use it, slit the plastic with a sharp knife, peel it back and grate it with a cheese grater. You may notice that the coconut oil has separated and congealed at the end of the package. As this had less of a coconut taste to it, it is best to grate the block lengthways. I find this is easier to do if I've stored it in the fridge.
You'll find creamed coconut in the World Foods or the West Indian Foods aisle of a supermarket.