You get them from completing the raidsodes infected being the only one available atm and the shop for them is coming in the next big patch although looking at the items there not much different from the exploration ticket items apart from an added 10% damage bonus against the borg in ground weapons and 7.5% against borg for ship weapons.
You get them for completing STF missions, that means raids at endgame (RA5 / BG 5 level). Currently there is only one STF mission - Infected, however soon another STF mission called The Cure should come.
You can buy nothing for Marks of Valor yet, the store that exchanges those Marks for items has yet to be implemented in-game. There are some news about the Store though that it should have ship and personal weapons that have some Borg damaging abilities.