You can use the in game store and the earth republic camp's vendor, as the last commenter said. However, in addition to the episode missions giving a couple of outfits, some regular missions give outfits and some are rewarded for side missions/challenges. But the hood, in particular, can be found in the Earth repub. camp. It's called the city survivalist or something like that. You can view any outfits that are available for sale at vendors or in the in-game store (defiance store) before you purchase them.
Also, some of the outfits and head gear change "looks" depending on your race. For example, there is a helmet you can buy in the in the Defiance store that will be a short top hat on the irathient race. The hood is missing on the gas mask from the defiance store, too, on the Irathients. At least for my irathient.
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