Western grocery stores generally don't carry lemon grass, you'll need to go to an Asian market. Many of them will also sell dried (powdered) lemon grass, which is definitively the best substitute you can find for fresh lemon grass.
Honestly, there's really very little else you can substitute. Lemon grass has this hint of citrus flavour but also, as the name implies, a sort of grassy, herb-like flavour.
If I were really desperate, I would substitute lemon zest (fresh only), at 1/2 tsp for each lemon grass stalk called for by the recipe, and several pages online seem to suggest adding in some arugula (to give it that grassy quality). But be warned, it is not going to be the same, it's going to be a great deal more bitter, so consider either reducing the quantity or adding more sweet/savoury spices or ingredients to the curry to compensate (cinnamon, perhaps).