Find a body shop suppliers in your area (yellow pages or ask your local garage) then take the car there and have it matched.
Your local Halfords can match paint from the code on the car, again take the car with you, but over time the paint will fade.
You can have the paint put into a rattle-can if you wish but I have got really sick of the things so I bought a cheap compressor and half-decent spray gun. If your car is metallic you reaally want to do the job properly, even an electric spray gun is better than aerosol and they start at around £16. \:D/