Tickets for the most popular shows in the West End sell out up to a year in advance so, to be sure of seeing any particular show, it's always best to book as far in advance as possible.
The official booking site is here:
http://www.ticketmaster.co.uk/venueartist/1481 99/957635/?brand=uk_solt&affiliate=SOLT
If you are seeking 'on the day' tickets in London, the best place to start is always the 'Tkts' booth in Leicester Square, where tickets for some shows are available at half price:
http://www.officiallondontheatre.co.uk/tkts/ab out_tkts/
Chris