I'm looking for the origin of the phrase "build it and they will
come".  I know the movie field of dreams made it famous, but I was
hoping to find the original source.

For what it's worth, I seem to recall that the exact quote in the
movie "Field of Dreams" (and--I think, but I'm not certain--in the
original novel "Shoeless Joe" on which the movie was based) is
actually, "If you build it, he will come."  Which is a little
different in meaning than the way the quote's evolved in the public
consciousness to be the more encompassing "...they will come."

Could it be that the expression goes back to the much older:

"Build a better mousetrap, and the world will beat a path to your door."
Ralph Waldo Emerson
http://www.brainyquote.com/quotes/authors/r/ralph_waldo_emerson.html

It would be hard to believe that the phrase in the question or any
similar to it would not have been inspired by Emerson's words.
I, for example, had no idea that "build a better mousetrap" was a
quotation, not just a folksaying.  But a search found it immediately,
and my recollection of it was letter perfect  - however I learned it.
Maybe the author of "Shoeless Joe" and/or the scriptwriter(s) thought
he/they were being original, but it seems very likely that he/they
also once heard Emerson's aphorism and had it stored in the
subconscious.