You can purchase online easily via TicketSource.
Please note their advice relating to Ticket Exchanges and Refunds:
Before placing a booking, carefully review your event and ticket selection. Once purchased, tickets cannot be exchanged or refunded unless the Event is cancelled, moved to another date, or if details of the Event are significantly changed after a booking is placed. In the event of such reversal of transaction being desired, you should contact the Event Organiser directly, as they may, at their discretion, agree to your wishes.
A fee is charged by Ticketsource for each transaction, regardless of the number of tickets purchased.
This fee is dependent on the value of the transaction and will show on your ‘shopping basket’ before payment is taken.
If you do not wish to pay a fee, please purchase them via the local shops indicated above.