Magnesium Anodes for Shaft

[welder]

I have not been able to find magnesium anodes for shafts of my boat - need 1 3/4" stream-line type shaft anodes - does anyone know where to find? have been able to find 6"x12" diver plate hull anode in magnesium but not the shafts

boat in fresh water (Minnesota) so magnesium is appropriate - suggestions welcome