Hi! Nam pla is very hard to find where I live. From what I have read it doesn't have a very fishy flavour - would a mix of soy sauce and oyster sauce + 1/2 fish boullion cube do the trick? I have tried this before and the result tastes good to me but I don't know what nam pla is supposed to taste like!
Nam Pla is Thai fish sauce which is made from fermented fish (usually anchovies). It is quite salty and does have a slightly unique flavour from the fermented fish. You could try looking for the Vietnamese version of fish sauce which is known as nuoc mam.
To substitute fish sauce it slightly depends on what type of dish you are using the fish sauce in. If it is being used in a dipping sauce then it is difficult to find an exact substitute as the underlying flavour of the fish sauce is needed. You can try using Worcestershire sauce which is also made from fermented anchovies but Worcestershire sauce (such as Lea & Perrins) also contains tamarind so it has a slightly sour and tangy note to it and would need to be added very carefully to a dipping sauce. If the fish sauce is being used more as a seasoning in a larger dish then soy sauce and a little extra salt can be used, or soy sauce and a few drops of Worcestershire sauce.