I agree with GuideDogSaint.
#2 is more common.
#1 is understandable, but less common.
There is a small difference in meaning.
#1 = In this case, the word "Finding" assumes that the blood donor exists.
#2 = In this case, the word "Looking" expresses that this is a search, and it is possible that they might not find a blood donor.
And if the donor is already located, then you could say "The blood donor has been found."
Good luck