SPOILER: After the first boss ..

[canakin]

You don't find all Valkyries in the Viking areas, also you really don't need to look for them hard, they are kind of conveniently placed so you happen to find them while you are doing your thing anyway. Also you will find many Viking troops later in the game and there is an item thingie named Mysterious Parcel that you can use to buy any troop anywhere (except level 5 ones I think).