For the less common ingredients - those immediately identifiable as used in molecular gastronomy - I used feedback and advice from members of the eGullet site, there's a thread devoted to the topic of sourcing ingredients for Modernist Cuisine. It basically came down to sourcing from either whipped cream supplies in the UK, or Willpowder in the US - I went with Willpowder because the shipping was cheaper. Both were very helpful with emails though, and each has a slightly different range. Willpowder stocks N Zorbit but not Gellan, Whipped Cream Supplies the other way around for example.
The Gellan in the kits is direct from CP Kelco.
The more common ingredients, such as the food acids, were sourced locally from a few different online wholesalers. While I could be wrong, I didn't think there would be much variation between citric acid suppliers, for example.
But if you're in the US then I can recommend Willpowder, but head over to eGullet and read the thread on suppliers. I'd post the link but as I'm writing this their servers are down for maintenance.