You can try calling them, or checking their FAQ on the web site. There's a pull down menu
(http://nationalexpress.metafaq.com/templates/nationalexpress/main/mainpage)
Through our Telephone Sales Centres. Just call 08705 808080 (International customers please dial +44 8705 808080). Our sales advisors will be able to take your order and send your travel ticket by post, email or text message, or they can arrange for collection of your ticket at a number of offices around the country. Please note that a booking fee is applicable when you book over the phone.
Alternatively, you can purchase your travel ticket from any National Express Outlet. We have around 1000 agents located all over the UK including all terminals at all the major international airports. Discount Coachcards can be issued while you wait and used immediately to purchase tickets at the discount rate.