I've tried www.nshispeed.nl and www.thalys.com but haven't had much success when they ask for my address. They seem to offer only delivery of tickets by mail only and require an address in the Netherlands.
We're planning to take the train on Sept. 6th and want to make reservations for the flexible youth fares.
How are you guys getting around this? Can we wait till we get to Leiden and buy it at the station (hopefully those tickets will still be available)?