there are no common codes. the honda dealer can tell you, you just need to provide the number on the owners manual for the NAV system. the code is based purely on the system's serial and model number, so no code other than the one they can give you will work.

now it if is an aftermarket device try calling the device's makers (ie, call alpine if you have an alpine nav system installed)

you should have some number that they gave you, you might have just misplaced it (or you bought it used and it wasnt supplied, in which case ask who you bought it from)