Substitute for lemon grass in curry dish
Well I would suggest if you really can not get it, then leave it out altogether.  It provides accents of flavour rather than a primary usually in most curry recipes, so it can be omitted.

If you want to substitute it, then you can use lemon or lime zest, possibly with some mint leaves to freshen it a little.  But the end result will not be the same, just enhanced in a similar fashion to that which lemongrass achieves.