This should get you to the ticket-ordering form: https://www.wayahead-secure.co.uk/g2005/orderform.asp?showcode=110000&pricetype=7&pricetype2=9&pricetype3=10&filler1=glastonbury
Edit: Tickets now seem to be sold out online, but there are apparently still some left through telephone bookings. However, tickets can be cancelled up to May 13th, so presumably there's a chance that some more may be made available between now and then. (This was a public information broadcast brought to you by way of Glastonbury Forum.)