If you've booked a package with Disney direct you can prebook it by telephone only. It costs £33 and you get a voucher when you check in to take to a shop which sells photopass+.
If you are going before July [15th I think] there is a discount voucher doing the rounds which takes 20 Euro off the standard price of 49.99 so works out better value. Shout if you want it and I'll hunt out the link to the voucher.