In reply to JamButty:
In Leicester Square usually the best place to go and get tickets (at least where I always go) is the Tckts booth which is just of the green part of the square in its own building.
http://www.tkts.co.uk/leicester-square/
It's the same place kmhphoto has suggested.
All the prices are under the What's on Sale section. This continually updates as the days go by with different performances.