Lemongrass is a common ingredient in good Thai food, but don't try to make do with the dried, packaged version; it's nowhere near as potent as the fresh product. You should be able to find fresh lemongrass at any Asian supermarket. Frozen lemongrass is almost as good, however.

For a uniquely Southeast Asian flavor, cook with galangal (blue ginger). It's related to ginger, but has a piney flavor with overtones of citrus. You can find it in most Asian and Thai food supermarkets, though you may have to make do with frozen galangal rather than fresh.

You don't have to go with fresh mushrooms for your Thai food, though they should be available in most supermarkets, Asian or otherwise. Dried mushrooms are usually cheaper and just as good for you, though some experts argue that they're actually more nutritious.