You can buy fondant in the cake decorating and baking section of the supermarket or if you go to a cake shop they have modelling paste which is like fondant but easier to work. The stuff I buy is called Flower Moulding Paste.

To make black and grey you will need black food colouring. The Wilton gel is my weapon of choice and it's available in almost all cake decorating shops and some shops that sell cooking stuff (like Whisk and The Essential Ingredient). You can also buy the brown as a food colouring instead of trying to mix your own.

Have a look at the Wilton site - it has heaps of useful tips and tricks for working with fondant.

My tip - wear some gloves when kneading the colour through. If you forget you will need Trillian's tip - use lemon juice to get the stains off your hands.