You generally find them in caches on the map when playing (highlighted beacons you can see from a fair distance). The super credits are automatically given to everyone when one person picks it up.

Technically, there's a way to farm these indefinitely by essentially replaying the same map. But you can't unlock the guns/capes/gear in each warbond without medals, which you can only get by doing missions.

Personally, I like the gameloop and how microtransactions are done enough that I don't bother with the "farm" since credits are so easy to gain overtime. The real grind is the medals and samples which you need a team to gather efficiently.