the very fact that you called them "nemo fish" means you arent ready. their proper name is the false percula clownfish. they are a saltwater fish, which automaticly reserves them for those with experience.
From an aquatics store. Whist your at it buy the tank, lid, stand, expensive protein skimmer and filter, air pump, heater, live rock and sand...that will set you back hundreds of pounds.
Alternatively watch the movie again and buy a Nemo soft toy. If you dont even know the proper name for the fish (note, its a clownfish), you clearly dont know how to care for them. They are expensive and difficult fish to care for. They are becoming endangered because of stupid kids pandering to the "Nemo" fad.