How do you get the endless starman in New Super Mario Brothers.?

What you're probably referring to is a system such as Action Replay being used to 'hack' the game's memory and tamper with its mechanics.

However, there is a series of ? blocks in World 7-3 which will each give you a Starman if you're still under the effect of one, thereby allowing you to chain them, and reap in the faint blue coins!