Found in areas from the Woods (fence) on-wards, especially Northeast Woods and earlier)
Good Locations (Please note there are no guarantees, but these are locations where they have been sighted)
Town Square 11.4 There are so many helmets. Try out, just an advice.
The Sewers 10.X is said to have safety helmets
Northeast Woods 12.4 is good for safety helmets, as well as 12.8.
Small Street USA 13 has also been said to provide more helmets.
Northwest Woods 14.2 also may have helmets.
Central Square 15.X is also quite generous with helmets.
Southeast Woods 8.3 has some helmets. Rare but it's happened a few times.
Farming for these is time consuming and there are no absolute guarantees; it might take lots of tries.