You need a basic airbrush kit and a compressor to get you started. Then probably a scalpel / sharp cutting device, liquid mask and paints.
The paints and liquid mask are available from Jon at RCS he has a thread on here
http://www.rcsgraphicworx.co.uk/
And for everything else you can't buy from Jon these people are probably the best and cheapest
http://www.everythingairbrush.com/index.html