How can I get a sniper for agent 47
Here is a list of all unlocks. Find the Snipers in the list and do what is required.
Generally you do mission mastery, which means finishing the missions while doing challenges. You can check them in the main or pause menu. You usually get a feel of them when you follow “Mission Stories”, which are guided opportunities that help you killing your target. While doing that you already fulfill a handful of challenges.