Tickets can be booked through this site. A booking fee of £1.25 per ticket is applied.
You won't be sent physical tickets, but will need to present proof of purchase (a printed receipt or a booking confirmation on your phone) at the venue to gain access to the show.
If the show isn't sold out you can buy tickets in cash on the door of the venue from 30 minutes before the show starts.
A number of the venues are selling advance tickets through their own box offices. See the venue page for their details.