40N03P is just an N-channel, 30V, 40A enhancement mode MOSFET with 0.017 ohm Rdson.
It should not be difficult to find a replacement; but it may depend on the circuit. Use the selector guide at any manufacturer or distributor.
Since it seems to still be manufactured, I want to ask again: why do you need a substitute?
And where is your location, and other criteria? Can you order online, for instance?