Ambar looks promising from the outside, but like many other "natural product" shops in Istanbul, the selection is a bit limited when it comes to specifically veg. food.
Having said that, it's better than most, and you can get Tofu and a number of interesting sauces and variations on coffee, olive paste etc. The rest of the store has the usual types of herbs/spices/natural produce and it may be interesting if you're not exactly looking for veg. snacks.
For a better selection, try one of the Macro supermarkets in Nisantasi, or the Ecolife or Balya health stores in Cihangir.Pros: Good location, Stocks Tofu and some other items
Cons: Limited selection