I would agree with the above posters that there really isnt a substitute for real fish sauce. However, if you are having trouble finding it there are recipes for "mock fish sauce" that you can use in a pinch:
simmer for 10 minutes: 1 can drained anchovies + 2 cloves bruised and peeled garlic + 1/2 teaspoon brown sugar + 1/4 teaspoon salt + 1 1/4 cups water, then strain.
simmer for 10 minutes: 1 part soy sauce + 4 parts anchovies
soak anchovies from one small can (about 14 fillets) in 1 cup water, strain, then add 2 tablespoons soy sauce.
Hope that helps...