The only other reliable method is buying from authorised Theatre Ticket Agencies.
Agencies are independent companies who work with theatres to sell tickets on their behalf.
The reason is that theatres can only reach so many clients at a time, and agencies often have their own special market -like corporate or incoming tourists. Agencies were also historically made to pay for tickets in advance thus taking the financial risk of unsold seats from the producer's bank balance to their own. Their profits now come from either adding their own service fee to the ticket price or from a commission paid on sales by the theatre. Legitimate agencies carry the 'STAR' Logo in their advertisements. This is a Circle containing a star with a tick in the centre.