Not sure whereabouts in the country you are, but there's a new Airbrush / paint shop opened up in Telford. Shropshire Airbrush Supplies. Seems well stocked when I visited..
Online shop too.. With all the paints Bigred suggested above.. (Not everything they have is listed on the website yet, they are just building things up. So give them a call if you need to)
http://shropshireairbrush.com/index.php/shop