The most common cause of the leak you describe is the upper timing cover seals. This cover is located above the alternator and mates to the front of the cylinder head. The leaking oil will run down behind the brackets you describe.
There is some bad news to this. The head gasket can leak oil from the same area.
Replacing the cover seals is not a major expense. Replacing the head gasket is.
However, come to think of it the chain tensioner is located behind the alternator bracket. The seal for the tensioner may be leaking. Not a major expense.
But wait! It could be that someone has already made the repair and did a slack job of cleaning away the oil.
As far as a leak preventer goes, I wouldn't use one if there was one on the market. Most of them are designed to either stop up an orifice or cause seals to swell. Neither are good IMO.