Alternatives to the build in microphone on the Nikon D3100 DSLR?

Before you ask, there isn't a jack for an external mic. I've been reading about using something else to record the audio, like a zoom h4n (I think?) but that's way out of my price range.

Any suggestions on cheaper alternatives/any other ways I can use a shotgun microphone for filming would be really appreciated as I audio is really the only thing bringing my quality of films down.

You could get a decent digital recorder. An old iriver mp3 player for example.