I understand that אונאה is אוסר once it hits a sixth, I just cannot seem to find where we get this amount from?
The idea of a sixth in brought in Shulchan Aruch CH:M 227-2 from the Rosh B:M 4-20, that chazal said that if the ona’ah is less than a sixth the person would be mochel it, but not if it is a sixth. As a side point it is controversial if the איסור אונאה applies even if it is less than a sixth. Although the money doesn’t have to be returned, as the person was mochel it, it still might be assur to do. See CH:M 227-3, 6.