One of the places it's addressed is the first halachic case of copyright infringement: in the mid-1500s, Rabbi Moshe of Padua (known as "Maharam Padava" in yeshiva circles) put a great deal of effort into typesetting and printing an edition of the Rambam; shortly thereafter, some non-Jewish printers copied his work with slight tweaks and began selling it. Rabbi Moshe wrote to his cousin, Rabbi Moshe Issreles (the Ramah), who responded firstly that they were employing illegal business practices, and additionally that it's a mitzva to buy from a Jew.
The shiur I heard on the subject (recorded several years ago in Kew Gardens Hills) concluded (and I'm afraid I don't recall the source here) that one should be willing to spend "a little more" to give his business to Jews; no good definition of what's called "a little more."
What if the Jewish and non-Jewish sellers have the same price, but the non-Jewish seller is significantly more convenient? Do we say time is money? The shiur quoted an opinion from over 100 years ago that you always must inconvenience yourself to buy from the Jew, and concluded that with our busy lives today where time=money, that's not as clear-cut.
Final caveat, heard from Rabbi Breitowitz: the preference of giving business to Jews is only where it's an identical product. But if it's between the equally-kosher Schmerel's Pizza and Pablo's Pizza (let's assume Schmerel is Jewish and Pablo isn't) and you significantly prefer the taste of Pablo's Pizza, you're not expected to buy something you don't like as much! (I think he then said if Schmerel's Pizza is in the red it would be a mitzva to give him the business.)
