looking for a substitute
What's the recipe you need it for? You could use lemon zest to get a similar citrusy perfume, but it won't be exactly the same. I've also heard that a combination of mint leaves, fresh ginger and lime juice is also a good substitute.
toful broc/carrots curry type dish with coconut milk.
Depending on where you live, your supermarket might have Lemongrass paste in the produce section.
It's pricy but keeps well. My Publix, Target and wal-mart has it. I find it easier to incorporate in some dishes like rice and has a great taste. Gourmet Garden is the brand name, it's in chilled section in produce near the jarred dressings. http://www.gourmetgarden.com/us/sitemaps
On the sitemap there's a store locator, I hope this help.