There are many different websites out there - believe me I've been trawling them all for tickets to Sister Act yesterday and the day before! The best ideas I found were on the Martin Lewis website - www.moneysavingexpert.com (type in theatre in the search engine). Ideas like using the train package deals etc makes visiting the theatre cheaper. Also it says on the website that some theatres have tickets for sale on the day for a lot less - but it will mean queueing up at 8am to get them! Give it a whirl, I saved £40!