The Graham Cracker is a mythical object, only known from its existence in North American cook books. In the wider world, there are a number of substitutes which can be used, depending on what precise effect is required. Generally speaking the best effect is achieved using a plain biscuit generally known in the British Isles as a Digestive such as a Hob Nob (an Australasian version of this biscuit is the Shredded Wheatmeal). Drier biscuits, such as the SAO or Cream Cracker may also serve, though they don't tend to soak up surrounding moisture very well. For a sharper result, you could even use the classic Ginger Nut.
Here is a recipe for a good basic Graham Cracker Crust.