The story is described in the Adi Parva of the Mahabharata. A king named Shwetaki had once conducted a Yagna (fire-ritual) for twelve years, and having consumed all the ghee (clarified butter) that was offered into the fire, Agni the fire god felt lethargic. So he consulted Brahma, who told him to consume the Khandava forest:
O exalted one, thou hast eaten, for twelve years, a continuous stream of sacrificial butter poured into thy mouth! It is for this that illness hath seized thee. But, O Agni, grieve not for it. Thou shalt soon regain thy own nature. I shall dispel this surfeit of thine and the time for it is even come. The dreadful forest Khandava, that abode of the enemies of the gods, which thou hadst of old once consumed to ashes at the request of the gods, hath now become the home of numerous creatures. When thou will have eaten the fat of those creatures, thou shalt regain thy own nature. Proceed thither in haste to consume that forest with its living population. Thou wilt then be cured of thy malady.
Agni tried to consume the forest, but he was thwarted by Indra, who kept using rainstorms to extinguish the flames, because of Indra's friendship with the Naga king Takshaka who lived there. So Agni went back to Brahma for advice, and Brahma told him that the famous sages Nara and Narayana had reincarnated on the earth as Arjuna and Krishna. So he approached Arjuna and Krishna, who happened to be near the Khandava forest at the time, and asked them for help. They were happy to oblige, but they requested weapons that would be commensurate to the task of fighting Indra. So Agni summoned Varuna the ocean god, who gave Arjuna the famous Gandiva bow and chariot of Chandra the moon god.
And more importantly for our purposes, Krishna received the Sudarshana Chakra:
And [Agni] then gave unto Krishna a discus with an iron pole attached to a hole in the centre. And it was a fiery weapon and became his favourite. Having obtained that weapon, Krishna also became equal to the task. [Agni] then, addressing Krishna, said, "With this, O slayer of Madhu, thou shalt be able without doubt to vanquish in battle even foes that are not human. With this weapon, without doubt, thou shalt be superior in battle to men and gods, and Rakshasas and Pisachas, and Daityas and Nagas. And thou shalt certainly be able with this to smite all. And, O Madhava, hurled by thee in battle at thy foes, this weapon will irresistibly slay the enemy and again come back into thy hands."
It also says this: "And the lord Varuna, after this, gave unto Krishna a mace, of name Kaumodaki, capable of slaying every Daitya and producing, when hurled, a roar like that of the thunder." I assume that's the Gada of Balarama.
Not to keep you in suspsense, Krishna and Arjuna succeeded in their endeavor, which is not surprising; as Sanjaya says in the Bhagavad Gita, " Wherever there is Kṛṣṇa, the master of all mystics, and wherever there is Arjuna, the supreme archer, there will also certainly be opulence, victory, extraordinary power, and morality."
On a side note, you also asked about Balarama's plow. Well, the Srimad Bhagavatam describes Balarama as a "haladhara", or plow-wielder, back when he was a kid in Vrindavana. So he must already have had the plow at the time of the fight with Kamsa. I think the only reason he didn't use it is that Kamsa's wrestling contest was a hand-to-hand combat event.