I ran Amsterdam in 2008 and tried to find it before that (& failed). Not sure if you can order it online anywhere but I was unsuccessful when trawling the sports shops in Holland during the previous summer.
In the event, I held off until about 20 miles before trying it and ended up finding it very palatable and easy on the stomach. Obviously everyone is different, but I have since found that I cannot tolerate Lucozade or Nuun electrolyte drinks while running, so I think that's a recommendation.
Good luck with the hunt. 