It's really not essential. The one I use contains chillis, black pepper, garlic, ginger, coriander, lemongrass, basil and lime oil; if you don't have a source of the spice, I'd substitute powdered coriander and ginger, and dried basil, and possibly lime and lemongrass, since the recipe has a lot of the rest of it anyway. Or just coriander. Or dried basil. Honestly, go wild. I tend to think of these things as being according to personal taste, anyway.