Forsaken Ones
You will be able to purchase these cards in the Store on Rewards / Faction tab after minimum Reputation is achieved to unlock the reward. Every faction does have an associated achievement to play all reward cards on a fixed mission and receive gold in reward. Most of the time player will get more gold back than amount spent to buy one copy of each of required cards. However, note that this is an one-time proposal only - 2nd and next cards will not get any refund.
0Assembly Plant0Vorpal Tank0Tactical Infiltrator

Associated achievements

Reputation gain

Trying to grind mission with highest reputation per energy gain is not always the best and the fastest way to raise reputation due to increased chance to lose. Most of the time it's better to step 1-2 missions back and grind there.
Reputation after completing all associated missions (once each): 814 / 2,000 (1,186 to max).
201 reputation at Mission 47, 814 reputation at Mission 53.
Mission 491.8311615.0090
Mission 501.7112715.00105
Mission 531.6715915.56140
Mission 511.6313815.00120
Mission 521.5614915.00135
Mission 471.139816.38131
Mission 481.1110915.00135
Mission 461.008815.62125
Mission 450.887815.00120
Mission 440.866715.00105
Mission 430.835615.8395
Mission 420.674615.0090
RPE - reputation per 1 energy spent, GPE - gold per 1 energy spent.