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Waat? Where does one get that outfit?!

You can purchase that from the PlayStation store, it doesn't cost much to buy the clothing pack for Chris Redfield and Sheva Alomar.

Check here at this link for more details --> https://www.gamefaqs.com/boards/929198-resident-evil-5/53622770 and here ---> https://www.gamefaqs.com/ps3/980166-resident-evil-5-gold-edition/answers/157488-mercenaries-reunion-dlc-

You will look for the extra add-on content, to unlock the Mercenaries Reunion gameplay option.