Are there sources that יצחק knew what was going on, during the `Akeda and that he was okay with it?
How would these sources come to this conclusion?
According to B'reishit Rabbah, Yitzchak not only knew but approved and cooperated. (All quotes here are from the Soncino translation.)
56:4 tells how Samael interfered, first by trying to dissuade Avraham and then turning to Yitzchak:
Seeing that he could achieve nought with [Avraham], he approached Isaac and said: ‘Son of an unhappy mother! He goes to slay thee.’ 'I accept my fate,’ he replied. 'If so,’ said he, 'shall all those fine tunics which thy mother made be a legacy for Ishmael, the hated of her house?’ If a word is not wholly effective, it may yet avail in part’; hence it is written, AND ISSAC SPOKE UNTO ABRAHAM HIS FATHER, AND SAID: MY FATHER: why HIS FATHER... MY FATHER? So that he should be filled with compassion for him. AND HE SAID: BEHOLD, THE FIRE AND THE WOOD. ‘May that man be drowned who has thus incited him,’ exclaimed he. At all events, GOD WILL PROVIDE HIMSELF THE LAMB, O my son; and if not, THOU ART FOR A BURNT-OFFERING, MY SON. ' SO THEY WENT BOTH OF THEM TOGETHER-one to slaughter and the other to be slaughtered.
So according to this passage, Samael told him and Avraham confirmed the rumor.
56:8 tells how Yitzchak was concerned that it be done properly (which he could only do if he knew):
R. Isaac said: When Abraham wished to sacrifice his son Isaac, he said to him: ‘Father, I am a young man and am afraid that my body may tremble through fear of the knife and I will grieve thee, whereby the slaughter may be rendered unfit and this will not count as a real sacrifice; therefore bind me very firmly. Forthwith, HE BOUND ISAAC: can one bind a man thirty-seven years old? (another version: twenty-six years old) without his consent?
Yitzchak knew, having first heard it from Samael and then from his own father. Rather than fleeing, he cooperated.
Rashi on 22:8 says their 'walking together' shows he was ok with it.
A rundown of those pesukim by request:
Verse 6 they walked together meaning both were equally happy. Avraham who knew what was to happen was as happy as Yitzchok in his blissful lack of knowledge of what was to transpire.
Verse 7: due to Avraham's actions, Yitchok gets suspicious and inquires where is the sheep?
Verse 8: Avraham says Hashem will choose one, which Yitzchok understands means he is on deck. They travel together meaning both were happy with one heart.
The Tzeror Hamor to Vayeira (22:8) says that would be impossible to that occurred without Yitzchok's cooperation. This is deduced from the fact that the word בני in the answer of Avraham to Yitzchok would suggest the following understanding: "God will see to the sheep for His burnt offering, (which is you) my son". The following statement ("they went both of them together") would prove that Yitzchok agreed with that. See Radak on 22:8 for a more elaborate explanation on this.