Does anyone know of any RotTMNT fan fiction where Leo seeks a mother figure and goes to Big Mama, who takes care of him without betraying him?

I'm not aware of any specific fan fiction that fits that description, but it sounds like an interesting and unique idea for a story! You may want to try searching on fan fiction websites or forums dedicated to RotTMNT to see if anyone has written a story like that. Alternatively, you could try writing your own fan fiction based on that premise.