Gemara

71a - Mishnah: A Kohen Hedyot serves with four garments, i.e. a Kesones, Michnesayim, Mitznefes and Avnet.

The Kohen Gadol wears these and four more, a Choshen, Efod, Me'il and Tzitz.

He wears these when someone asks from the Urim v'Tumim.

We ask only for the king, (Av - Gra deletes this) Beis Din or someone whom the Tzibur needs.

73b - Question: What is the source of this?

Answer (R. Avahu): "V'Lifnei Elazar...";

"Hu" refers to (Yehoshua, who will be) the king; "V'Chol Benei Yisrael Ito" refers to the Meshu'ach Milchamah (the Kohen that speaks to the soldiers in war); "V'Chol ha'Edah" refers to the Sanhedrin.

Shevu'os 14a - Mishnah: The following are required to add on to Yerushalayim or the Azarah - a king, a Navi, the Urim v'Tumim...

Sanhedrin 16a - Mishnah: Seventy-one judges are needed to decide to go to an optional war.

Question: What is the source of this?

Answer #1 (R. Avahu): "V'Lifnei Elazar...";

Rejection: Perhaps the verse does not refer to deciding to go to war. Rather, it teaches for whose needs we ask of the Urim v'Tumim!

Answer #2: Rav Acha bar Bizna taught that Chachamim told David that Yisrael cannot finance themselves. David authorized them to go to war. They took counsel with Achisofel and the Sanhedrin, and asked the Urim v'Tumim.

Rav Yosef: Rav Acha learns from "V'Acharei Achisofel Yehoyada ben Benayahu v'Evyasar v'Sar Tzava la'Melech Yo'av";

Yehoyada ben Benayahu was the head of the Sanhedrin, Evyasar would ask from the Urim v'Tumim - "U'Vnayahu ben Yehoyada Al ha'Keresi veha'Pelesi".

Rishonim

Rambam (Hilchos Klei ha'Makdish 10:10): In Bayis Sheni they made an Urim v'Tumim to complete the eight Begadim, even though they did not ask from it.

Source (Kesef Mishneh): Kidushin 31a says that in the days of Bayis Sheni they sought stones for the Ephod. SMaG says that there was a Choshen because the Kohen Gadol cannot be Mechusar Begadim, but they did not have Hash-m's name inside.

Rambam (Hilchos Beis ha'Bechirah 4:1): In Bayis Sheni they did not answer questions from the Urim v'Tumim with Ru'ach ha'Kodesh. They had it so the Kohen Gadol would not be Mechusar Begadim.

Pirush ha'Mishnayos: One may ask only through a Kohen Gadol wearing the eight Begadim.

Question (Mishneh l'Melech Klei ha'Makdish 10:11): A Beraisa teaches that we may ask from the Meshu'ach Milchamah. Why did the Rambam omit this?

Answer (Merkeves ha'Mishneh Klei ha'Makdish 10:11): Pirush ha'Mishnayos says that we may ask only from a Kohen Gadol. Also, the Meshu'ach Milchamah serves in the four Begadim. If he wore the eight Begadim, he would transgress Sha'atnez! The Beraisa means that the Meshu'ach Milchamah may ask from the Urim v'Tumim.

Rashash Yoma (73a DH Ki): One may ask even from a Kohen Hedyot if he has Ru'ach ha'Kodesh.

Rambam (Hilchos Beis ha'Bechirah 7:11): To add to the city (in which Kodshim Kalim may be eaten) or the Azarah we need a king, a Navi, the Urim v'Tumim and the Great Sanhedrin.

Ri Korkus and Tosfos (Shevu'os 15a DH she'Ein): The Mishnah in Sanhedrin mentions only that one must consult with the Great Sanhedrin to add to the city or Azarah. We expound that we must also ask the king, advisor and the Urim v'Tumim.

Ramban (Sefer ha'Mitzvos, eighth Aseh that the Rambam omitted): We must be wholehearted with Hash-m, and ask about the future only through His Nevi'im and the Urim v'Tumim.

Rashi (Eruvin 45a DH Harei): We do not ask Halachos of the Urim v'Tumim.

Mechilta (Shemos 22:7): We do not ask the Urim v'Tumim to decide a monetary case.

Amuk Davar (Shemos 28:15): Most questions of the Urim v'Tumim were for the sake of war.

Tosfos (Gitin 68a DH Zil): They did not want to ask the Urim v'Tumim (where to find the Shamir) because they were able to ask (the king of the Shedim) without it.