What is the origin of Krishna and of all other souls?
Krishna has no origin. He is anadi, without beginning. He is sarva karana karanam, the cause of everything, yet He has no cause. The living entities are also eternal. Krishna is their source, but they have the same spiritual nature and are therefore also eternal.
"Krishna, who is known as Govinda is the Supreme Godhead. He has an eternal blissful spiritual body. He is the origin of all. He has no other origin and He is the prime cause of all causes." ( Brahma-samhita, 5.1)
The following verse is also helpful for understanding this:
"The Personality of Godhead is pure, being free from all contaminations of material tinges. He is the Absolute Truth and the embodiment of full and perfect knowledge. He is all-pervading, without beginning or end, and without rival. O Narada, O great sage, the great thinkers can know Him when completely freed from all material hankerings and when sheltered under undisturbed conditions of the senses. Otherwise, by untenable arguments, all is distorted, and the Lord disappears from our sight.
I hope this is helpful.