There is only one official Half Price Ticket Booth in London, situated in the clocktower building adjioning the garden on the south side of Leicester Square. It's easy to find - it is the only building actually in the Square, as opposed to round the outside. The nearest Tube stations are Leicester Square and Piccadilly Circus.
Tickets are available for a wide selection of West End shows on the day of performance.
You cannot book ahead as the Booth only sells tickets for performances the same day. You can see which shows have tickets available by looking at the two noticeboards in front of the building. Check the list of shows outside for details. Sales are by cash or credit card only, cheques and travellers cheques are not accepted. Not all West End Shows have tickets on sale at the Half Price Ticket Booth - but there's always plenty of choice.