Are there any sources—and if so what are the sources—in the Rishonim and Acharonim that allow a person to get drunk on Purim day during the Seudas Mitzva?
Although many sources prohibit intoxication (Ramban Comm. to Kedoshim, Rambam Hil. Deos, Meoros Megillah 7b, Kol Bo and Orchos Chaim cited by Darchei Moshe and Beis Yosef respectively in Orach Chaim 695) some sources write hat one may or should get drunk on purim:
1) There is an obligation to get drunk (Rif to Megillah 7b, Piskei Rid, Piskei Riaz, and Tur.) Additionally Minhagim Tirna and Minhagim Kloizner write to get drunk without specifying if it is obligatory.
2) Drink until you cant compute the gimatriyos of baruch mordechai and arur haman (Aguda, R' Yerucham).
3) Drink only up to the point of ad dlo yada (it is ad vlo ad bichlaal): Meoros to Megillah 7b (in one tzad).
4) Drink until you sleep. (Rambam (Hil. Megillah 2:15) Maharam Mirotenburg (responsa).
5) Drink until you cant sing the baruch mordechai song (Tosafos according to Beis Yosef and many others).
6) Seder Hayom writes to get drunk until you pass out (the most extreme view I have seen)
7) M'chir Yayin of Rama (commentary to 9:19).