I always buy before we travel. Try
http://www.orlando-ticket-deals.co.uk/tickets/ they offer good prices.
It seems that you can only get 14 day unlimited Disney access tickets in the UK, the US versions I've found are 10 days maximum, so if you're looking for access to the parks every day for a fortnight, better buy before you go. (It may seem extreme to consider going into the parks every day, but the tickets cover you for the water parks and Downtown Disney as well, also it's nice to have a day out somewhere else and return to a park in the evening for the parades and fireworks. Finally, these unlimited tickets allow you to visit more than one park in a day, so you could for exakple, have the morning in Typhoon Lagoon, rest for the afternoon back at the hotel, then return to MK for a meal and fireworks.
I'll stop talking now...;-)
Have a good holiday - we're going in 4 weeks and 6 days and counting....