There are a number of sources in the UK, depending on exactly what you are looking for.
You can pick up a DIY compressor at places like Wickes and B&Q, but be prepared to wear ear defenders.
http://www.aircompressors.co.uk/oilfree.php
They are cheap though.
If you want something quieter you need to look to the specialists:
http://airbrushes.com
http://www.artifolk.co.uk/catalog/airbrushes_and_compressors.htm
http://www.ripmax.com/5index.asp?category=T1450&selectedtab=140
Karl