The general law:
The verse states “Vechi Timkaru Mimkar Leamisecha, Oa Kanah Miyad Amisecha Al Tono Ish Es Achiv/When you sell or purchase from your brother do not cheat him.” From this verse it is learned that a Jew is to support Jewish businesses when doing commerce. This means that one is to buy products from a Jew and sell products to a Jew.
Obligation or good advice? Some Poskim rule that the above statement is not an obligation, but simply good advice, and hence one may choose to buy/sell to a gentile over a Jew, if he so wishes. Majority of Poskim, however, rule that the above statement is an absolute obligation upon every Jew, and is not just a matter of piety, encouragement or good advice. [Practically, this Mitzvah is to be treated as an actual obligation.]
The detailed cases of obligation & The law if there are differences in price, convenience, or quality, between the Jew and gentile:
Coming up in part 2!
 See Sifra Behar 3, brought in Rashi 25:14; Admur Halva 1; Rambam Matanos Aniyim 10:17; Tashbatz 3:151; Shut Rama 10; Levush Y.D.D 159:1; Shalom Bayis 66 [Rebbe of M”A]; Chikrei Lev C.M. 139; Chasam Sofer C.M. 79 and 134; Pischeiy Teshuvah C.M. 156:6; Maharam Shick C.M. 31; Maharsham in Mishpat Shalom C.M. 189; Machaneh Chaim C.M. 2:24; Ahavas Chesed 5:6-7; Toefes Reim 22; Shearim Hametzuyanim Behalacha 1:62; Yaskil Avdi 4:6; Minchas Yitzchak 3:129; Teshuvos Vehanhagos 1:805; 2:724; Ateres Paz 1:10; Piskeiy Uziel 48
 Sifra [Toras Kohanim] Behar Parsha, [Omitted from Rambam, Tur and Shulchan Aruch] “From where do we learn that if one sells [a product] that he should only sell it to a colleague [i.e. Jew]? From the verse [Vayikra 25:14] “Ki Simkor Memkar Liamisecha” From where do we learn that if one buys [a product] that he should only buy it from a colleague [i.e. Jew]? From the verse [Vayikra 25:14] “Kaneh Miyad Amisecha” From where do we know that this applies not only to real estate, of which the verse was discussing, but also to all merchandise, as the verse says “Memkar,” which includes all items of purchase or sale.”; Rashi Vayikra 25:29; See also Avoda Zara 20a regarding selling Niveila meat that one is to precede selling to a Ger Toshav versus a gentile [from which it can be understood that certainly one is to precede selling to a Jew than to a gentile]; See also Admur Halva 1 and Bava Metzia 71a regarding lending to a Jew versus a gentile, that one is to precede to lend to a Jew for free rather than lend to a gentile for interest; See Pnei Yehoshua on Bava Metzia ibid that a) from the teaching brought in Avoda Zara ibid one can learn a general rule that it’s a Mitzvah to precede a Jew to a gentile, and b) from the ruling here in Bava Metzia, one can deduce that it is a Mitzvah to precede a Jew even in a case of substantial loss; Shut Rama 10, brought in Pischeiy Teshuvah C.M. 156:6, in 3rd reason/Yesod for prohibiting people from buying the Rambam set printed by the gentile and requiring them to buy from Maharam Padvah, that one must precede buying from a Jew versus a gentile; Tashbeitz 3:151 that one must sell to a Jew even if he is offering slightly less, and certainly if he is offering the same amount; Ahavas Chesed 5:6; Igros Kodesh 13:292, printed in Shulchan Menachem 7:32, “Its understood that you should travel whenever possible with the company El Al, as Chazal state on the verse “Kanah Miyad Amisecha”;
 Vayikra 25:14
 Pesikta Zutrasa Behar 71b; Levush Y.D. 159:1 that the above verse is a mere Asmachta; Chikrei Lev C.M. 139, based on the fact that this teaching of the Sifra is omitted from the Rambam, Tur and Shulchan Aruch; Maharsham C.M. 189; Yaskil Avdi 4:6-1 that this law only applies when all the Jewish people are living in Eretz Yisrael
 Shut Rama 10; Tashbatz 3:151; Implication of Chasam Sofer C.M. 79; Pischeiy Teshuvah C.M. 156:6; Maharam Shick C.M. 31; Ahavas Chesed 5:6; Minchas Yitzchak 3:129; So is evident from many of the Poskim recorded below who discuss difference in price, that in essence they agree that it is an obligation
 So is evident from majority of the Poskim ibid who discuss this issue. See Poskim in first footnote