http://www.dvla-som.co.uk/
http://www.regtransfers.co.uk/
http://www.newreg.co.uk/
http://www.britishcarregistrations.co.uk/
http://www.personallyyours.co.uk/

(Never used any - Personalised plates are just a bit low rent )
Bear in mind that some companies can be selling the same plate at the same time. Might be cheaper on another site.