If I remember, Shri Krishna and Balarama didn't have the weapons Sudarshana chakra and Hala respectively with them when they were fighting with Kamsa.
So I am assuming they got these weapons later. How and from where did they got these weapons and do they have other weapons as well?
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.
@Keshav Your answer describes how Krsna obtained his Sudarshana and Kaumodaki, which are of course his main weapons. But apart from these two, Srimad Bhagavatam describes how they got most of their divine weapons.
In Canto 10, Chapter 50, the story of Jarasandha's invasion of Mathura is described. Accordingly, Jarasandha, to avenge his son-in-law, Kamsa, put Mathura under siege with 23 akshauhinis (each akshauhini consisting of 21,870 soldiers on elephants, 21,870 charioteers, 65,610 cavalrymen and 109,350 infantry soldiers) at his command.
At this hour of crisis, Lord Krsna decided to wipe out the entire army of Jarasandha. When he decides thus, all his weapons and two chariots arrive from Vaikuntha to assist him.
SB 10.50.11 — As Lord Govinda was thinking in this way, two chariots as effulgent as the sun suddenly descended from the sky. They were complete with drivers and equipment.
SB 10.50.12 — The Lord’s eternal divine weapons also appeared before Him spontaneously. Seeing these, Śrī Kṛṣṇa, Lord of the senses, addressed Lord Saṅkarṣaṇa.
SB 10.50.13-14 — [The Supreme Lord said:] My respected elder brother, see this danger which has beset Your dependents, the Yadus! And see, dear master, how Your personal chariot and favorite weapons have come before You. The purpose for which We have taken birth, My Lord, is to secure the welfare of Our devotees. Please now remove from the earth the burden of these twenty-three armies.
SB 10.50.15 — After Lord Kṛṣṇa had thus invited His brother, the two Dāśārhas, Kṛṣṇa and Balarāma, wearing armor and displaying Their resplendent weapons, drove out of the city in Their chariots. Only a very small contingent of soldiers accompanied Them.
(You can read the whole chapter here).
So Krsna and Balarama acquired most of their weapons at this point. And additionally, Krsna also got the Sudarshana and Kaumodaki at the time of Khandava Dahana.
There are many stories in which it is said Shiva or Agni gifted Sudarshana Chakra to Krishna/Vishnu, but in reality it is not the case. All the weapons are Krishna's eternal property and his weapons immediately appear in front of him when he calls them or wish to use them. Just because, he received the weapon (s) from someone in his divine pastimes, it doesn't mean someone gifted it to him originally or it was not his own property.
Srila Prabhupada said:
Sudarśana: This wheel which is accepted by the Personality of Godhead (Viṣṇu or Kṛṣṇa) as His personal weapon is the most powerful weapon, greater than the brahmāstras or similar other disastrous weapons. In some of the Vedic literatures it is said that Agnideva, the fire-god, presented this weapon to Lord Śrī Kṛṣṇa, but factually this weapon is eternally carried by the Lord. Agnideva presented this weapon to Kṛṣṇa in the same way that Rukmiṇī was given by Mahārāja Rukma to the Lord. The Lord accepts such presentations from His devotees, even though such presentations are eternally His property. There is an elaborate description of this weapon in the Ādi-parva of the Mahābhārata. Lord Śrī Kṛṣṇa used this weapon to kill Śiśupāla, a rival of the Lord. He also killed Śālva by this weapon, and sometimes He wanted His friend Arjuna to use it to kill his enemies (Mahābhārata, Virāṭa-parva 56.3).
Ref: Purport of https://www.vedabase.com/en/sb/1/9/6-7
Hare Krishna!
